Showing papers in "Limnology and Oceanography in 2006"

The global abundance and size distribution of lakes, ponds, and impoundments

Abstract: One of the major impediments to the integration of lentic ecosystems into global environmental analyses has been fragmentary data on the extent and size distribution of lakes, ponds, and impoundments. We use new data sources, enhanced spatial resolution, and new analytical approaches to provide new estimates of the global abundance of surface-water bodies. A global model based on the Pareto distribution shows that the global extent of natural lakes is twice as large as previously known (304 million lakes; 4.2 million km2 in area) and is dominated in area by millions of water bodies smaller than 1 km2. Similar analyses of impoundments based on inventories of large, engineered dams show that impounded waters cover approximately 0.26 million km 2 . However, construction of low-tech farm impoundments is estimated to be between 0.1% and 6% of farm area worldwide, dependent upon precipitation, and represents .77,000 km 2 globally, at present. Overall, about 4.6 million km2 of the earth’s continental ‘‘land’’ surface (.3%) is covered by water. These analyses underscore the importance of explicitly considering lakes, ponds, and impoundments, especially small ones, in global analyses of rates and processes.

Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades

Abstract: The first special volume of Limnology and Oceanography, published in 1972, focused on whether phosphorus (P) or carbon (C) is the major agent causing eutrophication in aquatic ecosystems. Only slight mention was made that estuaries may behave differently from lakes and that nitrogen (N) may cause eutrophication in estuaries. In the following decade, an understanding of eutrophication in estuaries proceeded in relative isolation from the community of scientists studying lakes. National water quality policy in the United States was directed almost solely toward P control for both lakes and estuaries, and similarly, European nations tended to focus on P control in lakes. Although bioassay data indicated N control of eutrophication in estuaries as early as the 1970s, this body of knowledge was treated with skepticism by many freshwater scientists and water-quality managers, because bioassay data in lakes often did not properly indicate the importance of P relative to C in those ecosystems. Hence, the bioassay data in estuaries had little influence on water-quality management. Over the past two decades, a strong consensus has evolved among the scientific community that N is the primary cause of eutrophication in many coastal ecosystems. The development of this consensus was based in part on data from whole-ecosystem studies and on a growing body of evidence that presented convincing mechanistic reasons why the controls of eutrophication in lakes and coastal marine ecosystems may differ. Even though N is probably the major cause of eutrophication in most coastal systems in the temperate zone, optimal management of coastal eutrophication suggests controlling both N and P, in part because P can limit primary production in some systems. In addition, excess P in estuaries can interact with the availability of N and silica (Si) to adversely affect ecological structure. Reduction of P to upstream freshwater ecosystems can also benefit coastal marine ecosystems through mechanisms such as increased Si fluxes.

Recent advances in the understanding and management of eutrophication

Abstract: Major advances in the scientific understanding and management of eutrophication have been made since the late 1960s. The control of point sources of phosphorus reduced algal blooms in many lakes. Diffuse nutrient sources from land use changes and urbanization in the catchments of lakes have proved possible to control but require many years of restoration efforts. The importance of water residence time to eutrophication has been recognized. Changes in aquatic communities contribute to eutrophication via the trophic cascade, nutrient stoichiometry, and transport of nutrients from benthic to pelagic regions. Overexploitation of piscivorous fishes appears to be a particularly common amplifier of eutrophication. Internal nutrient loading can be controlled by reducing external loading, although the full response of lakes may take decades. In the years ahead, climate warming will aggravate eutrophication in lakes receiving point sources of nutrients, as a result of increasing water residence times. Decreased silica supplies from dwindling inflows may increasingly favor the replacement of diatoms by nitrogen-fixing Cyanobacteria. Increases in transport of nitrogen by rivers to estuaries and coastal oceans have followed increased use of nitrogen in agriculture and increasing emissions to the atmosphere. Our understanding of eutrophication and its management has evolved from simple control of nutrient sources to recognition that it is often a cumulative effects problem that will require protection and restoration of many features of a lake's community and its catchment.

Eutrophication of freshwater and marine ecosystems

Abstract: Initial understanding of the links between nutrients and aquatic productivity originated in Europe in the early 1900s, and our knowledge base has expanded greatly during the past 40 yr. This explosion of eutrophication-related research has made it unequivocally clear that a comprehensive strategy to prevent excessive amounts of nitrogen and phosphorus from entering our waterways is needed to protect our lakes, rivers, and coasts from water quality deterioration. However, despite these very significant advances, cultural eutrophication remains one of the foremost problems for protecting our valuable surface water resources. The papers in this special issue provide a valuable cross section and synthesis of our current understanding of both freshwater and marine eutrophication science. They also serve to identify gaps in our knowledge and will help to guide future research. Knowledge of the links between nutrients and aquatic productivity began with the pioneering work of Weber (1907) on German peat bogs and with Johnstone’s (1908) studies of the North Sea. A crystallization of freshwater eutrophication concepts took place soon thereafter in Northern Europe, where the first trophic classification systems for surface waters were developed. These early classification systems were based on the intensity of aquatic organic matter production, as well as nutrient supply conditions and ecosystemlevel consequences of increased production (e.g., hypolimnetic oxygen depletion; Rodhe 1969). There was a lot of uncertainty in the subsequent 50 yr about the physical, chemical, and ecological details of the eutrophication process, and hot debates raged about the relative roles of different mineral nutrients as constraints on, or regulators of, primary productivity, especially the macronutrients nitrogen (N), phosphorus (P), and carbon (C). Work on the eutrophication process accelerated in the 1960s and 1970s. Particularly important was the landmark 1971 American Society of Limnology and Oceanography (ASLO) eutrophication symposium that culminated in the publication of the first special issue of Limnology and Oceanography (L&O) on nutrients and eutrophication, edited by G. E. Likens (Likens 1972a). This special issue was similarly stimulated by a symposium that the three of us 1

Eutrophication and trophic state in rivers and streams

Abstract: Many natural streams are net heterotrophic, so I propose that trophic state be divided into autotrophic and heterotrophic state. This division allows consideration of the influence of external carbon sources as well as nutrients such as nitrogen and phosphorus. Empirical results suggest that phosphorus and nitrogen are the most important nutrients regulating autotrophic state in flowing waters and that benthic algal biomass is positively correlated to gross primary production in streams. Reference (minimally influenced by human activities) nutrient concentrations and correlations of nutrients with algal biomass are used to characterize reference distributions of stream autotrophic state. Only when reference nutrient concentrations are in the upper one third of those expected in the United States, is maximum benthic chlorophyll projected to exceed 100 mg m 22 (a concentration commonly used to indicate nuisance levels) .30% of the time. Average reference nutrient concentrations lead to sestonic chlorophyll concentrations above those considered typical of eutrophic lakes (. 8m g m 23 ) less than half the time. Preliminary analysis suggests that autotrophic state is variable in small pristine streams because it is influenced by canopy cover (light), but heterotrophic state is less variable because it can be based on allochthonous or autochthonous production. Nitrogen and phosphorus enrichment can influence both heterotrophic and autotrophic state, and these effects could cascade to animal communities. Stoichiometry should be considered because carbon, nitrogen, and phosphorus are all involved in trophic state. The proposed definition of trophic state offers a starting conceptual framework for such considerations.

Seaborne measurements of near infrared water-leaving reflectance: The similarity spectrum for turbid waters

Abstract: Theory and seaborne measurements are presented for the near infrared (NIR: 700‐900 nm) water-leaving reflectance in turbid waters. According to theory, the shape of the NIR spectrum is determined largely by pure water absorption and is thus almost invariant. A ‘‘similarity’’ NIR reflectance spectrum is defined by normalization at 780 nm. This spectrum is calculated from seaborne reflectance measurements and is compared with that derived from laboratory water absorption measurements. Factors influencing the shape of the similarity spectrum are analyzed theoretically and by radiative transfer simulations. These simulations show that the similarity spectrum is valid for waters ranging from moderately turbid (e.g., water-leaving reflectance at 780 nm of order 10 24 or total suspended matter concentration of order 0.3 g m 23 ) to extremely turbid (e.g., reflectance at 780 nm of order 10 21 ). Measurement uncertainties are analyzed, and the air-sea interface correction is shown to be critical for low reflectances. Applications of the NIR similarity spectrum to atmospheric correction of ocean color data and to the quality control of seaborne, airborne, and spaceborne reflectance measurements in turbid waters are outlined.

The effect of carbonate chemistry on calcification and photosynthesis in the hermatypic coral Acropora eurystoma

Abstract: The rise in atmospheric CO2 has caused significant decrease in sea surface pH and carbonate ion (CO 22 3 ) concentration. This decrease has a negative effect on calcification in hermatypic corals and other calcifying organisms. We report the results of three laboratory experiments designed specifically to separate the effects of the different carbonate chemistry parameters (pH, CO 22 3 ,C O2 [aq], total alkalinity [AT], and total inorganic carbon [CT]) on the calcification, photosynthesis, and respiration of the hermatypic coral Acropora eurystoma. The carbonate system was varied to change pH (7.9–8.5), without changing CT ;C T was changed keeping the pH constant, and CT was changed keeping the pCO2 constant. In all of these experiments, calcification (both light and dark) was positively correlated with CO 22 3 concentration, suggesting that the corals are not sensitive to pH or CT but to the CO 22 3 concentration. A decrease of ,30% in the CO 22 3 concentration (which is equivalent to a decrease of about 0.2 pH units in seawater) caused a calcification decrease of about 50%. These results suggest that calcification in today’s ocean (pCO2 5 370 ppm) is lower by ,20% compared with preindustrial time (pCO2 5 280 ppm). An additional decrease of ,35% is expected if atmospheric CO2 concentration doubles (pCO2 5 560 ppm). In all of these experiments, photosynthesis and respiration did not show any significant response to changes in the carbonate chemistry of seawater. Based on this observation, we propose a mechanism by which the photosynthesis of symbionts is enhanced by coral calcification at high pH when CO2(aq) is low. Overall it seems that photosynthesis and calcification support each other mainly through internal pH regulation, which provides CO 22 3 ions for calcification and CO2(aq) for photosynthesis. The increase in atmospheric CO2 is associated with global warming, rising sea level, and surface ocean acidification (Brewer 1997; Feely et al. 2004). Atmospheric CO2 is expected to double relative to its preindustrial level sometime between 2050 and 2100 according to different IPCC scenarios (Houghton et al. 2001). Under such conditions the calculated surface ocean pH will be 7.9 by the year 2060 compared with a value of ,8.2 during the preindustrial time (Brewer 1997). This decrease will cause a significant decline in the carbonate ion concentration (CO 22

Anthropogenic and climatic influences on the eutrophication of large estuarine ecosystems

Abstract: We examined the effects of anthropogenic and climatic perturbations on nutrient-phytoplankton interactions and eutrophication in the waters of the largest estuarine systems in the U.S.A., the Chesapeake Bay (CB), Maryland/ Virginia, and the Neuse River Estuary/Pamlico Sound (NRE/PS) system, North Carolina. Both systems have experienced large post-World War II increases in nitrogen (N) and phosphorus (P) loading, and nutrient reductions have been initiated to alleviate symptoms of eutrophication. However, ecosystem-level effects of these nutrient reductions are strongly affected by hydrologic variability, including severe droughts and a recent increase in Atlantic hurricane activity. Phytoplankton community responses to these hydrologic perturbations, including storm surges and floods, were examined and when possible, compared for these systems. In both systems, the resulting variability in water residence time strongly influenced seasonal and longer-term patterns of phytoplankton biomass and community composition. Fast-growing diatoms were favored during years of high discharge and short residence time in CB, whereas this effect was not observed during high discharge conditions in the longer residence time NRE/ PS. In the NRE/PS, all phytoplankton groups except summer cyanobacterial populations showed decreased abundance during elevated flow years when compared to low flow years. Although hurricanes affected the CB less frequently than the NRE/PS, they nonetheless influenced floral composition in both systems. Seasonally, hydrologic perturbations, including droughts, floods, and storm-related deep mixing events, overwhelmed nutrient controls on floral composition. This underscores the difficulty in predicting seasonal and longer-term phytoplankton production and compositional responses to nutrient input reductions aimed at controlling eutrophication of large estuarine ecosystems.

Anaerobic ammonium oxidation in the oxygen-deficient waters off northern Chile

Abstract: We investigated the pathways of N2 production in the oxygen-deficient water column of the eastern tropical South Pacific off Iquique, Chile, at 20uS, through short anoxic incubations with 15N-labelled nitrogen compounds. The location was characterized by steep chemical gradients, with oxygen decreasing to below detection at ,50-m depth, while nitrite reached 6 mmol L21 and ammonium was less than 50 nmol L21. Ammonium was oxidized to N2 with no lag phase during the incubations, and when only NH þ was 15N-labeled, 15N appeared in the form of 14N15N, whereas 15N15N was not detected. Likewise, nitrite was reduced to N2 at rates similar to the rates of ammonium oxidation, and when only NO 2 was 15N-labeled, 15N appeared mainly as 14 N 15 N, whereas 15 N 15 N appeared in only one incubation. These observations indicate that ammonium was oxidized and nitrite was reduced through the anammox reaction, whereas denitrification was generally not detected and, therefore, was a minor sink for nitrite. Anammox rates were highest, up to 0.7 nmol N2 L 21 h 21 , just below the oxycline, whereas rates were undetectable, ,0.2 nmol N2 L21 h21, deeper in the oxygen-deficient zone. Instead of complete denitrification to N2, oxidation of organic matter during the incubations may have been coupled to reduction of nitrate to nitrite. This process was evident from strong increases in nitrite concentrations toward the end of the incubations. The results point to anammox as an active process in the major open-ocean oxygen-deficient zones, which are generally recognized as important sites of denitrification. Still, denitrification remains the simplest explanation for most of the nitrogen deficiency in these zones.

Effects of cyanobacterial toxicity and morphology on the population growth of freshwater zooplankton: Meta-analyses of laboratory experiments

Abstract: We synthesized data from 66 published laboratory studies, representing 597 experimental comparisons, examining the effects of cyanobacterial toxicity and morphology on the population growth rate and survivorship of 17 genera (34 species) of freshwater, herbivorous zooplankton. Two meta-analyses were conducted with these data. The primary analysis compared herbivore population growth rates for grazers fed treatment diets containing cyanobacteria versus control diets comprising phytoplankton that are generally considered to be nutritious for zooplankton (chlorophytes and/or flagellates). This analysis confirmed that cyanobacteria were poor foods relative to small chlorophytes and flagellates. More importantly, filamentous cyanobacteria were found to be significantly better foods for grazers than single-celled cyanobacteria over all studies. Surprisingly, the presence or absence of commonly-measured toxic compounds (microcystins in 70% of the cases) in the diet had no overall influence on grazer population growth relative to control diets. A secondary analysis compared survival rates for grazers fed cyanobacteria versus no food. In contrast to the primary analysis, grazer survival was more negatively affected by toxic cyanobacteria than non-toxic cyanobacteria, relative to starvation. However, this difference was attributable to the effects of a single Microcystis strain, PCC7820. Thus, though some cyanobacterial strains appear to be toxic to some strains of zooplankton, the overall role of commonly-assayed cyanobacterial toxins as a determinant of food quality may be less than widely assumed. We suggest that more attention be focused on nutritional deficiencies, morphology, and the toxicity of undescribed cyanobacterial compounds as mediators of the poor food quality of cyanobacteria for zooplankton.

Nitrogen fixation and dissimilatory nitrate reduction to ammonium (DNRA) support nitrogen dynamics in Texas estuaries

Abstract: We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH ) flux ranged from slight uptake (negative values) 1 4 to NH production rates of about 300 mmol m 22 h 21 (units used for all N rates) and increased with salinity ( p 5 1 4 0.10). Net nitrate (NO ) flux ( 220 to 32) and net N2 flux ( 270 to 100) did not relate to salinity. Mean net N2 flux 2 3 was positive but near zero, indicating that N 2 sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO reduction to NH (DNRA) rates were estimated after inflow water was enriched 21 34 with 15 NO (100 mmol L 21 ). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 2 3 to 97. Potential DNRA, measured conservatively as 15 NH accumulation, ranged from 0 to 80 and related signifi1 4 cantly to salinity ( p , 0.01). Increases in total NH release after 15 NO additions were higher but closely related 12 43 (r 5 0.9998) to 15 NH accumulation, implying exchange reactions of DNRA-regenerated 15 NH with sediment1 1 4 4 bound 14 NH . The fate of NO was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was 12 43 high in southeastern Corpus Christi Bay in August during hypoxia when the sulfide transition zone was near the sediment surface. Nitrogen fixation and DNRA are important mechanisms that add and retain available N in Texas estuaries.

Consequences of the 2003 European heat wave for lake temperature profiles, thermal stability, and hypolimnetic oxygen depletion: Implications for a warmer world

Abstract: In summer 2003 central Europe suffered an unusually severe heat wave, with air temperatures similar to those predicted for an average summer during the late 21st century. We use a unique set of over half a century of lake data from two lakes in Switzerland to determine the effect of the 2003 heat wave on water temperature and oxygen conditions in order to assess how temperate lakes will react when exposed to the increased ambient summer air temperatures that will be encountered in a generally warmer world and to test the predictions of relevant simulation models. In both lakes, surface temperature and thermal stability in summer 2003 were the highest ever recorded, exceeding the long-term mean by more than 2.5 standard deviations. The extremely high degree of thermal stability resulted in extraordinarily strong hypolimnetic oxygen depletion. These results are consistent with the predictions of the simulation models. Additionally, the results indicate that climatic warming will increase the risk of occurrence of deep-water anoxia, thus counteracting long-term efforts that have been undertaken to ameliorate the effects of anthropogenic eutrophication.

Responses of estuarine and coastal marine phytoplankton to nitrogen and phosphorus enrichment

Abstract: A cross-ecosystem comparison of data obtained from 92 coastal zone ecosystems worldwide revealed a strong positive response of marine phytoplankton biomass to nutrient enrichment that is highly consistent with the general patterns reported previously in the limnological literature for freshwater lakes and reservoirs. Average concentrations of chlorophyll a in estuarine and coastal marine systems were strongly dependent on the mean concentrations of total nitrogen and total phosphorus in the water column. Moreover, as is true of freshwater ecosystems, the identity of the primary growth-limiting nutrient for marine phytoplankton appeared to be generally predictable from watercolumn total nitrogen : total phosphorus (TN : TP) ratios. This similarity in physiological response to nutrients likely derives from the shared evolutionary histories of marine and freshwater phytoplankton.

Daphnia fatty acid composition reflects that of their diet

Abstract: We conducted a series of experiments feeding Daphnia pulex nine different phytoplankton monocultures with widely varying fatty acid composition and nutritional values to test the extent to which Daphnia fatty acid composition was affected by diet. In general, Daphnia fatty acid composition matched that of their diet much more closely than it did the fatty acid composition of Daphnia consuming other diets. However, Daphnia had consistently less saturated fatty acids and more arachidonic acid than did their diet, and Daphnia consuming cyanobacteria had substantially less saturated fatty acids and more monounsaturated fatty acids than their diets. Daphnia that consumed cryptophytes, which are rich in v3 polyunsaturated fatty acids (PUFAs), had on average 47% 6 8% (61 SD) v3 PUFAs within their fatty acid pool, whereas Daphnia that consumed v3 PUFA–poor cyanophytes only had 6% 6 3% v3 PUFAs. The ratio of v 3t ov6 fatty acids in Daphnia was also strongly dependent on diet, and averaged < 10 : 1, 2 : 1, and 1 : 1 for Daphnia that consumed cryptophytes, chlorophytes, and cyanophytes, respectively. Furthermore, the sum of C20 and C22 v3 and v6 fatty acids in Daphnia was highly correlated with that of their diet (r 2 5 0.94). These results suggest analyses of Daphnia fatty acid composition may be a powerful means of inferring diet in the field. These results also suggest the nutritional benefits of consuming v3-rich phytoplankton will transfer up the food web, making zooplankton both more efficient at converting phytoplankton biomass to their own biomass as well as much more nutritious for the zooplanktivorous fish that consume them.

Prediction of velocity profiles and longitudinal dispersion in salt marsh vegetation

Abstract: To predict the behavior of solutes and suspended particles in wetlands, it is necessary to estimate advection and longitudinal dispersion. To better understand these processes, measurements were taken of stem frontal area, velocity, vertical diffusion, and longitudinal dispersion in a Spartina alterniflora salt marsh in the Plum Island Estuary in Rowley, Massachusetts. Vegetation volumetric frontal area peaked at 0.067 6 0.007 cm 21 near 10 cm from the bed. If the velocity profile in a dense emergent marsh canopy depends on the local balance between pressure forcing and vegetation drag, the velocity will vary inversely with canopy drag (i.e., velocity is minimum where the frontal area is maximum). In fact, the minimum velocity was observed at 10 cm from the bed. The momentum balance therefore provides a way to predict the velocity profile structure from canopy morphology. The vertical diffusion coefficient also depends on canopy characteristics, such that the vertical diffusion coefficient normalized by the velocity and stem diameter had a constant value of 0.17 6 0.08 at this study site. The canopy morphology also controls the longitudinal dispersion, observed in this study to be 4 to 27 cm 2 s 21

Regional climate change and harmful algal blooms in the northeast Atlantic

Abstract: We investigated long-term spatial variability in a number of Harmful Algal Blooms (HABs) in the northeast Atlantic and North Sea using data from the Continuous Plankton Recorder. Over the last four decades, some dinoflagellate taxa showed pronounced variation in the south and east of the North Sea, with the most significant increases being restricted to the adjacent waters off Norway. There was also a general decrease along the eastern coast of the United Kingdom. The most prominent feature in the interannual bloom frequencies over the last four decades was the anomalously high values recorded in the late 1980s in the northern and central North Sea areas. The only mesoscale area in the northeast Atlantic to show a significant increase in bloom formation over the last decade was the Norwegian coastal region. The changing spatial patterns of HAB taxa and the frequency of bloom formation are discussed in relation to regional climate change, in particular, changes in temperature, salinity, and the North Atlantic Oscillation (NAO). Areas highly vulnerable to the effects of regional climate change on HABs are Norwegian coastal waters and the Skagerrak. Other vulnerable areas include Danish coastal waters, and to a lesser extent, the German and Dutch Bight and the northern Irish Sea. Quite apart from eutrophication, our results give a preview of what might happen to certain HAB genera under changing climatic conditions in temperate environments and their responses to variability of climate oscillations such as the NAO.

Copepod biodiversity as an indicator of changes in ocean and climate conditions of the northern California current ecosystem

Abstract: We evaluated copepod taxonomic diversity as a potential biological indicator of ocean conditions in the northeast Pacific Ocean using data collected biweekly between May 1996 and December 2004 and from 1969 to 1973 and 1983 off Newport, Oregon During the summer, low copepod biodiversity is accompanied by high biomass, with the opposite patterns prevailing in the winter High biodiversity, and associated low biomass, is also observed during the summers of major El Nino events (1983, 1997-1998) and during years when the Pacific Decadal Oscillation is in a positive phase From 1996 to the present, seasonally adjusted monthly anomalies of taxa richness and the Shannon-Weiner diversity index were compared to hydrographic and meteorological variables and to basin-wide climatological indices Correlations of biodiversity with physical variables were strongest during the summer months for the Multivariate El Nino/Southern Oscillation Index, temperatures at a water depth of 50 m, and the Pacific Decadal Oscillation During a 4-year cool period (1999-2002), biodiversity was low, likely a result of the increased transport of coastal subarctic waters into the northern California current In recent years (2002-2005), however, there has been a dramatic increase in biodiversity These increases may be attributable to the influence of a weak El Nino event in 2003; yet, high biodiversity persisted through the summer of 2005 and at times was higher than during the strongest El Nino events of the 20th century (1983 and 1997- 1998) Our analyses suggest that changes in source waters, driven by remote basin scale forcing and not local environmental events, cause interannual-to-decadal variations in copepod biodiversity in the northern California current Numerous ecological studies have established the critical role that climate variability may play in controlling ecosystem dynamics in the north Pacific Some classic examples include the influence of El Nino cycles; the Pacific Decadal Oscillation (PDO); and climate regime shifts on

In situ fluxes and zonation of microbial activity in surface sediments of the Håkon Mosby Mud Volcano

Abstract: From the Hakon Mosby Mud Volcano (HMMV) on the southwest Barents Sea shelf, gas and fluids are expelled by active mud volcanism. We studied the mass transfer phenomena and microbial conversions in the surface layers using in situ microsensor measurements and on retrieved cores. The HMMV consists of three concentric habitats: a central area with gray mud, a surrounding area covered by white mats of big sulfide oxidizing filamentous bacteria (Beggiatoa), and a peripheral area colonized by symbiontic tube worms (Pogonophora). A fourth habitat comprised gray microbial mats near gas seeps. The differences between these four methane-fueled habitats are best explained by different transport rates of sulfate into the sediments and pore- water upflow rates. The upflow velocities were estimated by two independent methods at 3-6 m yr21 in the central area and 0.3-1 m yr21 in Beggiatoa mats. In the central area no sulfide was found, indicating that the rapidly rising sulfate-free fluids caused sulfate limitation that inhibited anaerobic oxidation of methane (AOM). Under Beggiatoa mats a steep sulfide peak was found at 2 to 3 cm below the seafloor (bsf), most likely due to AOM. All sulfide was oxidized anaerobically, possibly through nitrate reduction by Beggiatoa. The Beggiatoa mats were dominated by a single filamentous morphotype with a diameter of 10 mm and abundant sulfur inclusions. A high diversity of sulfide oxidizer morphotypes was observed in a grayish microbial mat near gas vents, where aerobic sulfide oxidation was important. The sediments colonized by Pogonophora were influenced by bioventilation, allowing sulfate penetration and AOM to 70 cm bsf. The HMMV is a unique and diverse ecosystem, the structure and functioning of which is mainly controlled by pore-water flow.

Forming the primary nitrite maximum: Nitrifiers or phytoplankton?

Abstract: As intermediary in a number of key biological processes, the dynamics of oceanic NO2- concentrations have historically been used as an indicator of the balance between oxidative and reductive pathways in the marine nitrogen cycle. As appreciation of the role of NO2- in the marine nitrogen cycle grew through the 1960s and 1970s, and data sets from different ocean basins became available, a common feature was observed in stratified water columns: a peak in NO2- concentrations at the base of the euphotic zone, with near zero concentrations both shallower and deeper. These concentrations are significant; they commonly range between 10 and 400 nmol L-1 but as high as 4,500 nmol L-1. This peak in NO2- concentration is termed the primary nitrite maximum (PNM). Since the 1960s, the mechanisms sustaining the ubiquitous PNM have remained uncertain, with available data supporting either bacterial nitrification or NO2- release by phytoplankton. Simple box models have reproduced the PNM feature with nitrification as the source of NO2-, whereas others have succeeded solely with phytoplankton. Conclusive identification of the mechanism(s) maintaining the PNM in the world's oceans has yet to be achieved, but the preponderance of data supports phytoplankton excretion, with nitrification likely playing only a supporting role. Furthermore, there are a number of potentially important inconsistencies in the role of nitrification between culture studies and field observations. Biological-physical interactions are likely also important in controlling PNM formation and maintenance.

Control by fiddler crabs (Uca vocans) and plant roots (Avicennia marina) on carbon, iron, and sulfur biogeochemistry in mangrove sediment

Abstract: The influence of mangrove saplings (Avicennia marina) and fiddler crabs (Uca vocans) on carbon, iron, and sulfur biogeochemistry in mangrove sediment was studied using outdoor mesocosms with and without plants (21 m22) and crabs (68 m22). Saplings grew more leaves and pneumatophores in the presence of crabs. Dense microalgal mats lead to two to six times higher benthic production and about two times higher benthic respiration in the absence of crabs. Particle mixing by crabs increased the reactive oxidized iron (Fe(III)) in the upper 2 cm of the sediment, whereas oxygen leaching by roots maintained the deeper rhizosphere oxidized and enriched in Fe(III). The highest microbial activity, measured as carbon dioxide production and iron reduction, occurred within the upper 2 cm of ungrazed sediment and was fueled by the large near-surface biomass of microalgae. Leaching of dissolved organic carbon (DOC) from roots stimulated bulk sulfate reduction and caused an upward cascading reduction of the sediment as indicated by low Fe(III) and high Fe(II) between 2-cm and 6-cm depth. The effect DOC was also evident as increased microbial abundance at all depths in the sediment. Fe(III) was the most important electron acceptor for microbial carbon oxidation in ungrazed sediment (63–70%), whereas sulfate reduction was more important in grazed sediment (36–44%), particularly in the presence of plants. Aerobic respiration always accounted for ,20%. Fiddler crabs and roots of A. marina have complementary effects on the biogeochemistry of mangrove sediment. Their association seems to be mutually beneficial with respect to growth and food availability.

Oxygen dynamics in the rhizosphere of Zostera marina: A two‐dimensional planar optode study

Abstract: The oxygen dynamics in the rhizosphere of Zostera marina was studied by use of planar optodes. Oxygen leakage to the rhizosphere was restricted to the root tip and extended only up to ;8 mm up along the root. The oxic sediment volume around the roots increased linearly with irradiance in the interval of 0‐250 mmol photons m 22

Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems

Abstract: Notes Limnol. Oceanogr., 51(1), 2006, 343–348 q 2006, by the American Society of Limnology and Oceanography, Inc. Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems Abstract—Using radium (Ra) isotopes and nutrient analy- ses, we found that submarine groundwater discharge (SGD) is an important source of ‘‘new’’ nutrients, particularly nitrogen, to coral reefs around the world. Nitrogen input estimates as- sociated with SGD range from 3 to 800 mmol h 21 per meter of shoreline. The use of Ra isotopes allows us to quantify the inorganic nitrogen input from this source of nutrients. Increas- ing coastal population and land use practices may enhance anthropogenic nutrient loading from submarine groundwater contributing to reef degradation. The relationship between nutrient dynamics and produc- tivity in coral reef systems has received considerable atten- tion. The impetus for this is the contrast between the high productivity and biomass of these systems and the typically clear, nutrient-poor surface waters bathing them (Marsh 1977; D’Eliaet al. 1981). In addition to efficient recycling of nutrients (Dubinsky 1990), coral reefs derive new nutri- ents by effective acquisition of both particulate and dis- solved sources from seawater impinging on the reefs; vig- orous water circulation and flow (Larend and Atkinson 1997); nitrogen (N) fixation (Weibeet al. 1975); and dust deposition (Hingaet al. 1991). Terrestrially derived input from submarine groundwater discharge (SGD) has been indicated as an important source of nutrients to coastal systems in general (Corbett et al. 1999; Umezawa et al. 2002; Garrison et al. 2003) and to coral reefs in particular (Valiela et al. 1990; Paerl 1997). However, it has been notoriously difficult to track non–point source groundwater as it moves into coastal seas and to de- scribe the interactions between fresh groundwater and sea- water at the land–sea interface (Burnett et al. 2002). Ac- cordingly, direct measurements of SGD to coastal reef systems have not been extensive, and the role of SGD as a source of nutrients to coral reef ecosystems has not been extensively and quantitatively determined. Discharge of groundwater into the sea is widespread; it occurs anywhere that an aquifer is connected hydraulically with the sea through permeable sediments or rocks and where the aquifer head is above sea level. Submarine groundwater flows into the coast at the interface between freshwater and seawater (the mixing zone) where the uncon- fined aquifer outcrops at the beach (Reay et al. 1992). To- ward the seaward edge of the mixing zone, water is brackish as a result of intrusion of salt water through permeable aqui- fer mixing as well as wave and tidal pumping (Li et al. 1999). The chemistry of the water in the mixing zone is altered such that it is chemically different than either the terrestrial freshwater or seawater components (Church 1996). This area has been referred to as the subterranean estuary (Moore 2003). Accordingly, the term SGD, as used in this article, does not refer to freshwater (meteoric) input but rather includes the freshwater-recirculated seawater mix- ture that is discharging at the coastline (Buddemeier 1996). Over the last few decades, Moore and collaborators have pioneered the use of the quartet of naturally occurring ra- dium (Ra) isotopes as tracers for ocean mixing and saline submarine groundwater input to coastal systems (Krest and Harvey 2003). The divalent cation Ra isotopes are bound to soil particles and rocks in freshwater. They readily desorb via ion ex- change in the presence of solutions of higher ionic strength (Webster et al. 1994; Yang et al. 2002). Accordingly, in coastal aquifers, where seawater with high ionic strength mixes and interacts with freshwater and aquifer rocks, waters enriched in Ra are observed (Moore 2003). Open seawater, on the other hand, has very low or constant Ra activities. Therefore, excess Ra (over the open seawater activities) in- dicates a coastal source that, in many cases, is due to SGD. Ra isotopes are excellent tracers for the study of SGD and mixing in coastal systems because of the distinct difference in activities between the end-member sources (e.g., open ocean and terrestrial saline waters) and because they behave conservatively after leaving the aquifer (accounting for ra- dioactive decay). In addition, the use of Ra isotopes has advantages over other techniques used for quantifying SGD related fluxes, since it allows for temporal and spatial inte- gration over the mean-life of the radionuclides (Moore 2003), and the different timescales of decay are useful as mixing tracers. Indeed, Ra isotopes have been extensively used to determine the discharge of freshwater, nutrients (Ca- ble et al. 1996; Krest et al. 2000; Kelly and Moran 2002), and other dissolved constituents to the coastal ocean (Shaw et al. 1998). However, SGD-associated nutrient supply to coral reef systems has not been fully evaluated using Ra isotopes. Recently, concern that fringing reefs are degrading through human effects, particularly as a result of increases in terrestrial-derived inputs of nutrients and sediments, has been raised (Wilkinson 1999). Therefore, evaluating and, particularly, quantifying the contribution of SGD-associated nutrient input to fringing reef systems assume critical im- portance. Methods—To determine if submarine groundwater is dis- charging at the beach and reaching the reefs, water samples were taken along several transects from the water line to some distance offshore (typically within 100 m from the shoreline) at representative fringing reef sites around the world (Fig. 1). Other sites (Mexico, Heron Island in Austra- lia) were also examined, and preliminary results are consis- tent with the data presented here; however, these data are not included because only a limited number of samples were collected. Only one representative transect is shown in the

Effects of eutrophication, grazing, and algal blooms on rocky shores

Abstract: Eutrophication can profoundly change rocky shore communities. These changes often cause the replacement of perennial, canopy-forming algae such as Fucus spp. with annual, bloom-forming algae such as Enteromorpha spp. Grazing, however, can counteract eutrophication by eliminating the annual algae’s susceptible recruits. We examine these generalizations across large scales. We use replicated ‘‘bioassay’’ experiments to compare the effects of eutrophication and grazing across four paired control versus eutrophied sites in the Northwest Atlantic and four eutrophied sites in the Baltic Sea in spring and summer. At each site, annual algal recruitment and grazing pressure were estimated using tiles seeded with Enteromorpha intestinalispropagules. Tiles were exposed for 3 weeks with grazers excluded or allowed access. Productivity of E. intestinalis recruits was strongly related to eutrophication (10-fold increase) and grazing (80% decrease) and was weakly related to season. While the absolute grazing rate increased in a linear fashion with algal productivity, the relative grazing rate remained surprisingly constant ( ;80%). Comparative field surveys showed that perennial algae decreased by 30‐60%, while annual algae, filter feeders, and grazers increased across a gradient of eutrophication. As eutrophication increased from control to eutrophied to point source sites, rocky shore communities became increasingly dominated by single species of annual algae or filter feeders, and community diversity declined consistently by 24‐46%. We conclude that grazers are important controllers of algal blooms but that, ultimately, they cannot override the effects of increasing eutrophication on rocky shore community structure and biodiversity. Rocky shores are among the most dynamic and productive ecosystems on the planet. Biomass and primary productivity are typically dominated by canopy-forming perennial macroalgae such as fucoids and laminarians. Together with seagrasses on soft-bottom habitats, these algae generate up to 40% of the primary productivity of the coastal zone (Char

Copper‐dependent iron transport in coastal and oceanic diatoms

Abstract: We investigated the presence of plasmalemma-bound copper-containing oxidases associated with the inducible iron (Fe) transport system in two diatoms of the genus Thalassiosira. Under Fe-limiting conditions, Thalassiosira oceanica, an oceanic isolate, was able to enzymatically oxidize inorganic Fe(II) extracellularly. This oxidase activity was dependent on copper (Cu) availability and diminished by exposure to a multi-Cu oxidase (MCO) inhibitor. The rates of Fe uptake from ferrioxamine B by Fe-limited T. oceanica were also dependent on Cu availability in the growth media. The effects of Cu limitation on Fe(II) oxidation and Fe uptake from ferrioxamine B were partially reversed after a short exposure to a Cu addition, indicating that the putative oxidases contain Cu. Limited physiological experiments were also performed with the coastal diatom Thalassiosira pseudonana and provided some evidence for putative Cu-containing oxidases in the high-affinity Fe transport system of this isolate. To support these preliminary physiological data, we searched the newly available T. pseudonana genome for a multi-Cu-containing oxidase gene and, using real-time polymerase chain reaction (PCR), quantified its expression under various Fe and Cu levels. We identified a putative MCO gene with predicted transmembrane domains and found that transcription levels of this gene were significantly elevated in Fe-limited cells relative to Fe-replete cells. These data collectively suggest that putative MCOs are part of the inducible Fe transport system of Fe-limited diatoms, which act to oxidize Fe(II) following reductive dissociation of Fe(III) from strong organic complexes.

Presence and regulation of alkaline phosphatase activity in eukaryotic phytoplankton from the coastal ocean: Implications for dissolved organic phosphorus remineralization

Abstract: The biologically important constituents of the dissolved organic phosphorus (DOP) pool, their bioavailability, and their cycling in coastal systems are still poorly understood. Here we use the enzyme alkaline phosphatase as a metric of DOP bioavailability and track the activity of this enzyme in a coastal system. We observed alkaline phosphatase activity (APA) in the >0.2-μm size fraction of all surface samples tested during an Oregon coast cruise in August 2001. Although there was not a significant trend between APA and phosphate concentration in the data set as a whole, chlorophyll a-normalized APA was elevated at the station with the lowest dissolved inorganic phosphate (DIP) concentration. Activity was also elevated in nutrient-addition experiments in which nitrate amendments were Used to force community drawdown of DIP. These data are consistent with phosphate regulation of APA. A cell-specific APA assay revealed that the percentage of diatoms with APA mimicked the trend in the hydrolytic rate, but such a trend was not observed for the dinoflagellates. Further, the percentage of dinoflagellate cells with APA was routinely higher than the percentage of diatom cells with activity. In nutrient-addition experiments designed to evaluate the regulation of APA, diatom taxa expressed APA less frequently than dinoflagellates, but they displayed a tighter regulation of the activity by DIP than dinoflagellates. The variability observed in the presence and regulation of APA in these eukaryotic phytoplankton indicates that DOP bioavailability is a potential driver of phytoplankton nutrition and species composition in the coastal ocean.

Bacterial Release of Dissolved Organic Matter During Cell Growth and Decline: Molecular Origin and Composition

Abstract: Heterotrophic bacterial growth and the chemical composition of dissolved organic matter (DOM) produced by bacteria from freshwater and marine environments were monitored during experiments with artificial media containing glucose as the sole carbon source. Glucose was quickly consumed, and DOM was released during bacterial growth. Percentages of extracellular release of DOM from bacteria ranged from 14% to 31%, indicating that bacterial production and growth efficiency are underestimated when only cellular carbon is measured. Relatively high concentrations of D-alanine (D-Ala) were observed in DOM released during exponential growth, whereas the concentrations of muramic acid and other D-amino acid components of peptidoglycan were not detected or were in low concentration. The selective release of D-Ala occurred during cell growth and division when peptidoglycan is cleaved and newly synthesized subunits are incorporated into the cell wall via transpeptidation. Most of the D-Ala released during exponential growth was rapidly consumed. Following exponential growth, bacterial abundance decreased due to grazing and possibly viral lysis. The DOM remaining in the incubations after one or more months included a mixture of D-amino acids commonly found in peptidoglycan and the amino sugars glucosamine and galactosamine, which were highly resistant to decomposition. The percentage of D-amino acids was much higher in DOM than in cells due to the preferential release of D-amino acids and decomposition of L-amino acids. The final concentrations of dissolved organic carbon (DOC) ranged from 20 to 30 mmol L21 regardless of the initial concentration of glucose or the source of inoculum. The observed

A bio-oceanographic filter to larval dispersal in a reef-building coral

Abstract: Gene flow was shown to be limited between western and eastern Caribbean populations of the reef-building coral, Acropora palmata. However, some mixing was detected among populations near Puerto Rico. Our genetic analyses categorize A. palmata samples from the east coast of the Dominican Republic with the western Caribbean population, suggesting a filter to gene flow east of the Dominican Republic. To test the hypothesis of a present day bio-oceanographic filter occurring between Puerto Rico and the Dominican Republic (i.e., in the Mona Passage), we used a Lagrangian stochastic model (LSM) of larval dispersal, coupling coral life history characteristics with physical forcing. The model operated at two spatial scales: Caribbean-wide and focusing on the Mona Passage area. Results from the Caribbean-wide study showed no significant virtual larval exchange between the two populations. The small-scale model indicated that virtual larvae do not readily traverse the Mona Passage during the corals' reproductive season. Larvae released from Mona Island, in the center of the passage, are retained in the lee within topographically steered eddies, which act, together with the larval competency period, as a de facto filter to dispersal. Combined, our findings reveal the location of a seasonal filter to gene flow and its mechanism.

Biodegradable dissolved organic matter in a temperate and a tropical stream determined from ultra-high resolution mass spectrometry

Abstract: We investigated dissolved organic matter (DOM) metabolism by employing plug-flow biofilm reactors and ultra‐ high resolution mass spectrometry of DOM isolated by C18 extraction in two forested stream ecosystems, a low DOM tropical stream sampled at baseflow and a higher DOM temperate stream sampled during a storm. On passage through the bioreactors, DOM concentrations in the tropical stream sample declined by 22%, whereas they declined by 42% in the temperate stream sample. The extracted DOM was subjected to electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry to obtain information on molecular weight distributions and elemental compositions for the thousands of compounds whose masses are calculated with sufficient accuracy to allow calculation of unique elemental formulas. In both streams, metabolism modifies DOM to lower molecular weight molecules, and oxygen-rich molecules are selectively biodegraded. Applying van Krevelen analyses for the unique elemental formulas of DOM constituents revealed that hydrogen-deficient molecules with low H : C ratios (assigned to black carbon‐derived molecules) are present and generally not metabolized. Black carbon molecules are refractory to biodegradation compared with other components of DOM, supporting the suggestion that black carbon molecules in DOM flow to the ocean without experiencing significant microbial degradation.

Phytoplankton photoacclimation and photoadaptation in response to environmental gradients in a shelf sea

Abstract: Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14 C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section (sPSII) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1/tPSII), the chlorophyll-specific maximum rate of carbon fixation (P ), and the light-saturation intensity ( Ek), all decreased from the surface to the subsurface chlorophyll * maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance. Photoacclimation describes the phenotypic response of algae to changes in irradiance at the organism level (Falkowski and LaRoche 1991) and can be assessed by measuring dif

Methanotrophic activity in relation to methane efflux and total heterotrophic bacterial production in a stratified, humic, boreal lake

Abstract: We studied methanotrophic activity in the water column in relation to heterotrophic bacterial production and efflux of methane (CH 4) from the lake surface in a small, stratified, humic, boreal lake (Valkea-Kotinen, southern Finland). During summer and winter stratification, the highest methanotrophic activities were in the metalimnion, where oxygen concentration was ,6 mmol m 23 . During an incomplete spring turnover and summer stratification period, 3‐5 times more CH4 was consumed by methanotrophs in the water column than was released to the atmosphere. The highest CH4 effluxes (1.2‐5.1 mmol m 22 d 21 ) to the atmosphere occurred during the autumnal turnover despite observed methanotrophic activity in the whole water column. In winter, the amount of CH4 consumed by methanotrophs (0.20 mol CH4 m 22 during 6.5 months) was of the same order of magnitude as that during the ice-free period (0.22 mol CH4 m 22 during 5.5 months). Annually ;80% of CH4 diffused from the sediment was consumed by methanotrophs in the water column, and only 20% (0.11 mol CH 4 m 22 yr 21 ) was released to the atmosphere. During the ice-free period, bacterial production measured as [ 14 C]leucine uptake showed a bell-shaped relation to CH4 concentration. The highest production was found in the metalimnion at CH4 concentrations ranging from 5 to 10 mmol m 23 . During summer stratification, net production of methanotrophs corresponded to 23‐47% of total bacterial production, but during the autumn turnover, this proportion was higher (27‐81%), indicating that methanotrophs offer a potentially significant source of carbon to zooplankton in stratified humic lakes.