Showing papers in "Limnology and Oceanography in 1999"

Synergism and antagonism among multiple stressors

Abstract: This study was designed to test for synergism (increased stress) or antagonism (decreased stress) among multiple environmental stressors using additive, multiplicative, and simple comparative effects models. Model predictions were compared to empirical results of laboratory experiments measuring interactions among thermal stress, toxin exposure, and low food on reproduction and survival of two species of cladoceran zooplankton. Stress was defined operationally as a reduction in reproduction or survival relative to optimal conditions over a 7-d period. These experiments are particularly applicable to episodic stresses such as those associated with short-term heat waves. Toxin or low food in combination with 30°C temperatures were generally more harmful than high temperature alone. However, most multiple stress effects were antagonistic, in that effects in combination were not as severe as predicted based on the sum or the product of their individual effects. In rare cases, interaction among stressors even diminished effects of the worst single stressor. Optimal conditions for reproduction and survival occurred at 25°C, high food and 0 mg liter ' toxin (a surfactant, sodium dodecyl sulfate). Suppressive effects of stressors examined individually ranked: high temperature (30°C) > SDS (10 mg liter -1 ≥ low food (∼100 μg C liter -1 ) > low temperature (20°C). Daphnia pulex isolated from a pond which experiences high summer temperatures throughout was more tolerant of 30°C conditions than Daphnia pulicaria isolated from a lake with a cold-water refuge. Differences were observed in individuals exposed as either adults or as 24-h neonates.

Multiple stressors on coral reefs: a long-term perspective

Abstract: Coral reefs are subject to a high frequency of recurrent biological and physical disturbances. The temporal and spatial scales of these are often large and difficult to study, so that most of our knowledge of disturbances on coral reefs comes from investigations conducted at one or a few sites, over short periods of time. We argue that studying single events in isolation can be misleading and that a longer term approach is necessary for understanding the responses of coral reef assemblages to multiple stressors. We present first a brief review of the impacts of physical disturbance (e.g., cyclones, hurricanes) on the community dynamics of coral reefs, with special attention to the effects of recurrent events. We then examine two unusually detailed, long-term data sets from Heron Island, Australia, and Jamaica which demonstrate some of the complexities of multiple stressors (broadly defined as natural or man-made disturbances). Both case studies illustrate that the effect of a particular disturbance often depends critically on the impact of previous perturbations. Consequently, even the same type of recurrent stressor can have different effects at different times, depending on history. Accordingly, when the added dimension of time is considered, the distinction between single and multiple stressors becomes blurred. Even a single event such as a hurricane can be viewed mechanistically as a multiple stressor, with short- and long-term impacts. We emphasize that multiple stressors often have significant effects on recruitment and regenerative processes of assemblages. These impacts are much less obvious than catastrophic or chronic mortality, but they play a crucial role in community dynamics over longer time scales. Importantly, chronic anthropogenic impacts can impede the ability of coral assemblages to recover from natural disasters, even where there is little detectable effect on rates of adult mortality. Once a reef has been degraded, it is usually impossible to ascertain retrospectively the precise mechanisms that were involved or the relative importance of different events. A single survey will provide a snapshot of the status of coral reefs, but a longer term approach is required to understand the processes underlying changes in assemblages.

Critical depth and critical turbulence: Two different mechanisms for the development of phytoplankton blooms

Abstract: A turbulent diffusion model shows that there are two different mechanisms for the development of phytoplankton blooms. One of these mechanisms works in well-mixed environments and corresponds to the classical critical depth theory. The other mechanism is based on the rate of turbulent mixing. If turbulent mixing is less than a critical turbulence, phytoplankton growth rates exceed the vertical mixing rates, and a bloom develops irrespective of the depth of the upper water layer. These results demonstrate that phytoplankton blooms can develop in the absence of vertical water-column stratification.

Linking diagenetic alteration of amino acids and bulk organic matter reactivity

Abstract: Examination of amino acids in particulate samples from a variety of marine environments (fresh phytoplankton to deep-sea sediments) revealed systematic compositional changes upon progressive degradation. These consistent trends have been used to derive a quantitative degradation index (DI) that is directly related to the reactivity of the organic material, as indicated by its lability to enzymatic decay and its first-order degradation rate constant. This direct link between molecular composition and degradation rate allows us to quantify the quality of organic matter based solely on its chemical composition.

Non-indigenous species as stressors in estuarine and marine communities: Assessing invasion impacts and interactions

Abstract: Invasions by non-indigenous species (NIS) are recognized as important stressors of many communities throughout the world. Here, we evaluated available data on the role of NIS in marine and estuarine communities and their interactions with other anthropogenic stressors, using an intensive analysis of the Chesapeake Bay region as a case study. First, we reviewed the reported ecological impacts of 196 species that occur in tidal waters of the bay, including species that are known invaders as well as some that are cryptogenic (i.e., of uncertain origin). Second, we compared the impacts reported in and out of the bay region for the same 54 species of plants and fish from this group that regularly occur in the region’s tidal waters. Third, we assessed the evidence for interaction in the distribution or performance of these 54 plant and fish species within the bay and other stressors. Of the 196 known and possible NIS, 39 (20%) were thought to have some significant impact on a resident population, community, habitat, or process within the bay region. However, quantitative data on impacts were found for only 12 of the 39, representing 31% of this group and 6% of all 196 species surveyed. The patterns of reported impacts in the bay for plants and fish were nearly identical: 29% were reported to have significant impacts, but quantitative impact data existed for only 7% (4/54) of these species. In contrast, 74% of the same species were reported to have significant impacts outside of the bay, and some quantitative impact data were found for 44% (24/54) of them. Although it appears that 20% of the plant and fish species in our analysis may have significant impacts in the bay region based upon impacts measured elsewhere, we suggest that studies outside the region cannot reliably predict such impacts. We surmise that quantitative impact measures for individual bays or estuaries generally exist for ,5% of the NIS present, and many of these measures are not particularly informative. Despite the increasing knowledge of marine invasions at many sites, it is evident that we understand little about the full extent and variety of the impacts they create—singly and cumulatively. Given the multiple anthropogenic stressors that overlap with NIS in estuaries, we predict NIS‐stressor interactions play an important role in the pattern and impact of invasions.

Dissolved organic carbon and nutrients as regulators of lake ecosystems: Resurrection of a more integrated paradigm

Abstract: The primary interpretive paradigm used to study lakes is their trophic status. Oligotrophic lakes have low nutrient loading and low productivity, while eutrophic lakes have high nutrients and high productivity. The strong empirical relationship between nutrient loading and productivity is a valuable tool for teaching, for research, and for management of lakes. In order to incorporate the variety of other known anthropogenic impacts on lakes, however, lake characterization needs to extend beyond the nutrient-productivity paradigm. For example, acid precipitation, heavy metal and toxic organic contaminants, increases in UV radiation, and global warming are all recognized threats to lake ecosystems. One of the key characteristics of lakes that determines how they respond to disturbances such as these is their concentration of colored dissolved organic carbon (CDOC). Here we argue that a paradigm that includes CDOC (using the absorption coefficient at 320 nm as a proxy) as well as nutrients will be useful in predicting and understanding the response of lake ecosystems to multiple stressors. We propose to resurrect the CDOC axis that was proposed by investigators earlier this century and to extend it by adding some operational definitions to permit placing some of the major lake types on the axes in a way that will help us to better understand the structure, function, and response to disturbance of lake ecosystems that are subject to natural and anthropogenic environmental changes at the local, regional, and global scales. Data from a few diverse lakes and a successional sequence in Glacier Bay, Alaska, are used to illustrate the potential utility of the 2-axis model in separating lake types.

How environmental stress affects the impacts of parasites

Abstract: Parasites occur in nearly every population. They often interact in complex ways with other stressors. In some cases, the interaction may lead to a disproportionately negative effect on the host population. In other cases, the stressor may ameliorate the effects of parasitism. Here we illustrate intersections of four types of environmental stressors with infectious diseases. First, pollutants may increase parasitism by increasing host susceptibility or by increasing the abundance of intermediate hosts and vectors. Pollutants can also decrease parasitism if infected hosts suffer differentially high mortality, parasites are more susceptible to pollution than their hosts, or if pollutants negatively affect intermediate hosts or vectors. These effects vary depending on the particular parasite and pollutant that interact. Second, habitat alterations such as impounding water or development can affect both intermediate host and vector populations such that the abundance of their attendant parasites is either increased or reduced. Third, fisheries can impact populations already stressed by disease. However, they may act to lower the density of a host population below the threshold for sustained transmission to such an extent that the parasite population can no longer persist. Fourth, introduced species may introduce new diseases to susceptible native populations or they may gain an advantage if they invade without the parasites from their native range. The complexity and ubiquity of these interactions are good arguments for considering parasitism when evaluating stressors of aquatic systems.

Photophysiology of the marine cyanobacterium Prochlorococcus: Ecotypic differences among cultured isolates

Abstract: Cultured isolates of Prochlorococcus from the Mediterranean Sea (MED4) and Sargasso Sea (SS120) have been shown to have dramatically different pigment composition and growth rate responses when grown over a range of irradiances. Moreover, analyses of field populations in the North Atlantic have shown that distinct ecotypes can coexist in the same water column. These and other observations have led to the hypothesis that Prochlorococcus is comprised of genetically distinct ecotypes that collectively expand the range of light intensities over which the genus can thrive. In this paper, we explore this hypothesis by comparing the photophysiology of 10 different Prochlorococcus isolates from diverse oceanographic regimes. We found that the 10 isolates could be grouped into two loose clusters based on their growth response to varying light intensity and their chlorophyll b/a2 (Chl b/a2) ratios. Although both groups photoacclimate when grown over a range of light intensities, isolates with distinctly higher Chl b/a2 ratios (high B/A ecotype) reach maximal growth rates at lower irradiances (Ik,g), have high growth efficiencies (ag), and are inhibited in growth at irradiances where isolates with low Chl b/a2 ratios (low B/A ecotype) are growing maximally. High Chl b/a2 ratios resulted in higher spectrally weighted average Chl a2-specific absorption coefficient (a ¯ ), Chl a2-specific light-harvesting efficiency (chla), and quantum yield (fm) under low growth irra* diances for the isolates of the high B/A ecotype relative to the others. The distinction between the high and low B/A ecotypes is supported by molecular phylogenies constructed using the 16S ribosomal ribonucleic acid (rRNA) gene. The physiological differences between the ecotypes most likely result in different relative distributions in a given water column and in fluctuations in their relative abundances as a function of seasonal dynamics and watercolumn stability.

Temperature regulation of nitrate uptake: A novel hypothesis about nitrate uptake and reduction in cool-water diatoms

Abstract: Marine diatoms generally form large blooms during periods of cool temperature (,208C), high NO fluxes ( .25 2 3 mM-N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO uptake kinetics and direct short-term temperature manipulation studies 2 3 on field diatom-dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO uptake by a Rhizoselenia-dominated population did not appear to saturate even at concen2 3 trations as high as 180 mM-N. We observed contrasting patterns of NO , NH , and urea utilization as a function 21 34 of experimental temperature (ambient 6 98C). Over the temperature range of 7‐258C, absolute uptake rates of NO (rNO32) decreased an average of 46% with increasing temperature from 7 to 258C (nine individual experiments), 2 3 while rNH41 and rUREA increased with increasing temperature by an average of 179 and 86% (eight individual experiments), respectively. Based on these observations and the nature of the physical environment, we hypothesize that these diatom-dominated populations were taking up NO in excess of nutritional requirements, the reduction 2 3 of which may serve as a sink for electrons during transient periods of imbalance between light energy harvesting and utilization. We suggest that the increase in non-nutritional NO uptake increases proportionately with the 2 3 magnitude of the imbalance between light energy harvesting and imbalance. This hypothesis reconciles previous observations of low C : N uptake ratios, high release rates of dissolved organic nitrogen or NO by diatom-dominated 2 2 assemblages, other observations of nonsaturating NO kinetics in field populations, and the apparent ‘‘preference’’ 2 3 for NO by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative 2 3 relationship between NO uptake and short-term temperature shifts, have important ecological implications. The 2 3 hypothesized ability of these diatom-dominated populations to better modulate the flow of photosynthetic electron energy, via NO reduction, in variable environments may provide a competitive advantage to diatoms and could 2 3 potentially explain why diatoms frequently dominate in regions of cool temperature, high NO flux, and turbulent 2 3 mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO uptake. 2 3 Finally, if a significant fraction of NO uptake is regulated by non-nutritional mechanisms in the cell, and if some 2 3 fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO , NH , or dissolved 21 24 organic nitrogen (DON), then estimates of new production based solely on NO uptake could be seriously biased. 2 3

Effects of agriculture, urbanization, and climate on water quality in the northern Great Plains

Abstract: The Qu'Appelle Valley drainage system provides water to a third of the population of the Canadian Great Plains, yet is plagued by poor water quality, excess plant growth, and periodic fish kills. Fossil algae (diatoms, pigments) and invertebrates (chironomids) in Pasqua Lake were analyzed by variance partitioning analysis (VPA) to determine the relative importance of climate, resource use, and urbanization as controls of aquatic community composition 1920-1994. From fossil analyses, we identified three distinct biological assemblages in Pasqua Lake. Prior to agriculture (ca. 1776-1890), the lake was naturally eutrophic with abundant cyanobacterial carotenoids (myxo-xanthophyll, aphanizophyll), eutrophic diatoms (Stephanodiscus niagarae, Aulacoseira granulata, Fragilaria capucina/bidens), and anoxia-tolerant chironomids (Chironomus). Principal components (PCA) and dissimilarity analyses demonstrated that diatom and chironomid communities did not vary significantly (P > 0.05) before European settlement. Communities changed rapidly during early land settlement (ca. 1890-1930) before forming a distinct assemblage ca. 1930–1960 characterized by elevated algal biomass (inferred as β-carotene), nuisance cyanobacteria, eutrophic Stephanodiscus hantzschii, and low abundance of deep-water zoobenthos. Recent fossil assemblages (1977–1994) were variable and indicated water quality had not improved despite 3-fold reduction in phosphorus from sewage. Comparison of fossil community change and continuous annual records of 83 environmental variables (1890–1994) using VPA captured 71–97% of variance in fossil composition using only 10-14 significant factors. Resource use (cropland area, livestock biomass) and urbanization (nitrogen in sewage) were stronger determinants of algal and chironomid community change than were climatic factors (temperature, evaporation, river discharge). Landscape analysis of inferred changes in past algal abundance (as β-carotene; ca. 1780-1994) indicated that urban impacts declined with distance from point sources and suggested that management strategies will vary with lake position within the catchment.

Major flux of terrigenous dissolved organic matter through the Arctic Ocean

Abstract: High-latitude rivers supply the Arctic Ocean with a disproportionately large share of global riverine discharge and terrigenous dissolved organic matter (DOM). We used the abundance of lignin, a macromolecule unique to vascular plants, and stable carbon isotope ratios (δ13C) to trace the high molecular weight fraction of terrigenous DOM in major water masses of the Arctic Ocean. Lignin oxidation products in ultrafiltered DOM (UDOM; >1,000 Da) from Arctic rivers were depleted in syringyl relative to vanillyl phenols (S/V = 0.3–0.5) compared to UDOM in temperate and tropical rivers (S/V = 0.5–1.2), indicating that gymnosperm vegetation is a major source of terrigenous UDOM to the Arctic Ocean. High concentrations of lignin oxidation products (83–320 ng L−1) and a depletion of 13C (δ13C = −23.0 to −21.9) in UDOM throughout the surface Arctic Ocean indicate that terrigenous UDOM accounts for a much greater fraction of the UDOM in the surface Arctic (5–33%) than in the Pacific and Atlantic oceans (0.7–2.4%). In contrast, UDOM in deep water from the Arctic Ocean as well as waters from throughout the Greenland Gyre had relatively low concentrations of lignin oxidation products (24–45 ng L−1 ) and was enriched in 13C (δ13C = −21.0 to −20.8). Terrigenous UDOM has a relatively short residence (∼1–6 yr) in surface polar waters prior to export to the north Atlantic Ocean. Assuming that the bulk of Arctic-derived DOM is compositionally similar to the UDOM fraction, we estimate that 12–41% of terrigenous DOM (2.9–10.3 Tg C yr−1 ) discharged by rivers to the Arctic Ocean is exported to the North Atlantic via surface waters of the East Greenland Current. It appears very little terrigenous DOM from the Arctic is incorporated into North Atlantic Deep Water and distributed globally via deep thermohaline circulation.

Effects of water velocity on algal carbon isotope ratios: Implications for river food web studies

Abstract: We used variation in algal d 13 C between river habitats to study the spatial scale of energy flow through river food webs. We found a strong negative relationship between herbivore d 13 C (which reflects algal d 13 C) and water velocity in three productive Northern California rivers but not in unproductive streams. The contrast among habitats suggests that water velocity affects algal d 13 C most strongly when CO2 availability is low relative to photosynthetic rates. Our results help explain the wide variation in published river biota d 13 C and show that past studies using carbon isotope analyses may have significantly underestimated the importance of algal-derived carbon to river food webs. While flow-related variation in d 13 C complicates this common application of carbon isotope analysis, we show that it provides a natural tracer of the flux of algal production derived from different habitats within rivers to higher trophic levels. Measurements of consumer d 13 C showed that most invertebrate and vertebrate consumers relied on local production, except for filter-feeding insects and steelhead trout, which relied on production derived from multiple sources. Stable carbon isotopes may thus be used to spatially delineate the habitats that support river food webs, providing previously unavailable information for understanding and managing river ecosystems. River food webs are based on both local microalgal production and carbon transported from terrestrial or upstream aquatic ecosystems. Determining the sources of production for river food webs is a major challenge to river ecologists because of the complexity of controls over carbon sources and the mobility of consumers (Schlosser 1991; Cummins et al. 1995). Stable carbon isotopes ( 13 C/ 12 Co rd 13 C) hold much promise as a tool for determining the energy base of river and stream food webs because terrestrial and aquatic plants often have different ratios of stable carbon isotopes (Rounick and Winterbourn 1986), and there is relatively small isotopic fractionation associated with trophic transfer of organic carbon (DeNiro and Epstein 1978; France 1996a). Use of carbon isotopes in streams and rivers, however, has been limited by unexplained variability in autotrophic d 13 C

The contribution of macrophyte‐derived organic matter to microbial biomass in salt‐marsh sediments: Stable carbon isotope analysis of microbial biomarkers

Abstract: Stable carbon isotope ratios of bacterial biomarkers were determined to infer sources of organic carbon used by bacteria in the sediments of three salt marshes. Biomarkers studied were polar lipid-derived fatty acids (PLFA), mainly bacteria-specific, methyl-branched i15 : 0 and a15 : 0. Experiments showed that isotopic fractionation between substrate and biomarkers was relatively constant (2 4t o26‰, on average) compared to the wide range in 13 C/ 12 C ratios of carbon sources found in the studied marshes. At the Spartina site of the Waarde Marsh (The Netherlands), biomarker 13 C/ 12 C ratios were depleted by approximately 6‰ more than expected for bacteria growing on Spartina litter and were similar to an unvegetated control sediment. This pattern suggested that local macrophyte production was of little importance and that other material (probably of algal origin) was the dominant carbon source for bacterial growth. Spartina contributed about half of the carbon in bacterial PLFA at the Kattendijke Marsh (The Netherlands) and dominated at the Great Marshes (U.S.). The variation in bacterial carbon sources in these marshes was probably related to estimated inputs of nonmacrophyte organic matter to the sediment. At the Waarde Marsh, a clear plant species effect was found as coupling between plant and bacteria was more important in Scirpus maritimus than in Spartina anglica. The contribution of local plant production to bacterial biomass in salt-marsh sediments is highly variable between marshes and depends on the input of nonmacrophyte material by sedimentation in comparison to local plant input, which in turn may differ among plant species.

Changes of the plankton spring outburst related to the North Atlantic Oscillation

Abstract: Changes in the timing, composition, and intensity of freshwater phytoplankton blooms are known to have an impact on water quality and aquatic ecosystem functions. Factors provoking these changes are, therefore, of major importance. In Lake Erken in southeastern Sweden considerable changes in the timing and large variations in the composition of phytoplankton spring peaks have been observed during the past 45 yr. Here we show that long-term changes and variations in Lake Erken are strongly related to a single global parameter-the North Atlantic Oscillation (NAO). Even regional parameters that are known to have most influence on the spring development of phytoplankton such as ice break-up and nutrient concentrations could not provide a more conclusive explanation of the observed changes in spring phytoplankton, making the NAO a very powerful and simple tool in determining the timing and composition of phytoplankton spring peaks in a temperate lake.

Patterns of groundwater discharge into Florida Bay

Abstract: Natural chemical tracers of groundwater discharge ( 222 Rn and CH4) were surveyed to evaluate possible patterns of groundwater interactions with surface water in Florida Bay. Radon and methane concentrations in water samples collected from wells, solution holes, canals, and Florida Bay showed a significant correlation, despite the fact that these two trace gases have independent sources. Groundwater flux was also measured directly via seepage meters in several Florida Bay locations. Natural abundance of nitrogen isotopes measured on attached algae and seagrass showed the greatest 15 N enrichment near the Keys. Collectively, our results suggested greater groundwater flow along the bay side of the Florida Keys. Nutrient flux estimates, based on interstitial nutrient concentrations and groundwater flux measurements, suggested that groundwater in the eastern part of Florida Bay may provide as much nitrogen (110 6 60 mmol N m 22 yr 21 ) and phosphate (0.21 6 0.11 mmol PO4 32 m 22 yr 21

The nutrient stoichiometry of benthic microalgal growth: Redfield proportions are optimal

Abstract: Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C: N: P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C: N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C: N ratio ranged uniformly around 7.5. N: P ratios 22 indicated P limitation. Under P limitation, the C: P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C: N: P = 119 : 17 : 1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106 : 16 : 1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.

Boundary mixing and nutrient fluxes in Mono Lake, California

Abstract: Temperature-gradient microstructure and nutrient profiling were undertaken at both an inshore and an offshore site on Mono Lake, California, to determine whether boundary mixing occurred and the effects on nutrient flux within the lake. Turbulence, as quantified by rates of dissipation of turbulent kinetic energy, was two to three orders of magnitude higher at the inshore site where the pycnocline intersected the bottom than at the same depths at an offshore station. The intense turbulence primarily occurred within 3.5 m of the sediment‐water interface. In addition, temperature profiles were more incrementally stepped in the pycnocline inshore than offshore. The Turner angle indicated that double-diffusive processes may have augmented turbulent transport in the upper 10 m, where temperatures were inversely stratified, but not in the main pycnocline. Within the pycnocline, e exceeded the threshold value for buoyancy flux ( ethr 5 15yN 2 ) in 21% of the turbulent layers inshore but in only 1% of the layers offshore. The coefficient of vertical eddy diffusivity, Kz, was two to four orders of magnitude higher withi n4mo f thebottom inshore than offshore at the same depths. Spatially averaged values of Kz, obtained from the heat-flux method using data obtained from both conductivity‐temperature‐depth (CTD) profiles and moored thermistor chains, were two orders of magnitude less than those obtained nearshore with microstructure profiling. From the differences in Kz, we inferred that most heat flux occurred due to boundary mixing at the base of the pycnocline inshore with the heat redistributed laterally by advection. Boundary mixing was initiated after winds were strong enough for the Lake number to decrease to a value of 2; thermocline compression and steepening of internal waves at the base of the pycnocline occurred, followed by packets of high-frequency internal waves critical for wave breaking. Calculated ammonium fluxes at the inshore site were sufficient to support daily rates of primary productivity in the deep chlorophyll maximum throughout the lake. These results indicate the vertical flux of nutrients across the nutricline in Mono Lake occurs over a limited area during intense mixing events initiated by high winds.

Circulation and larval distribution in internal tidal bore warm fronts

Abstract: Internal tidal bore warm fronts were observed during the summer of 1996 off the coast of Southern California. Warm bore fronts had concentrating currents resulting from high-frequency internal motions and from a larger two-way flow; the two-way flow featured surface currents onshore and bottom currents offshore. A sharp thermocline depression and high-frequency, large-amplitude internal motions followed the leading edge of the bore, with downwelling currents on the trailing side of the crest of the nonlinear internal waves and upwelling currents in front of the crest. Warm bores propagated onshore with a propagation speed, c, that ranged from 10.6 to 19.6 cm s -1 , while time-averaged frontal currents, ū, varied from 11.2 to 17.6 cm s -1 in the shallowest bin. In one out of three cases ū > c, which implied that there were faster currents than the rate of advance of the front and which implied that the origin of surface frontal material is behind the front, not in front of it. Three invertebrate larval taxa were found at all sites across fronts, but only two intertidal barnacles, Pollicipes polymerus and Chthamalus spp., were concentrated at the front's surface, while the subtidal bryozoan Membranipora spp. was not. Frontal Pollicipes were more concentrated than were Chthamalus. The frontal downwelling currents observed suggested that concentrated larvae would have to swim upward in order to maintain depth. Pollicipes were abundant on the offshore warm side of the fronts but were absent or rare on the onshore colder side, suggesting that the origin of frontal Pollicipes was behind the front, although an alternative cannot be ruled out conclusively. Chthamalus were more abundant at depth than at the surface at all sites except at the front, where this pattern was reversed. The origin of frontal Chthamalus is uncertain. Membranipora were more abundant on the onshore colder side of the fronts, and abundances were usually higher at depth than at surface. Lack of accumulation in this species may be due to its limited swimming capability.

Mesozooplankton influences on the microbial food web : Direct and indirect trophic interactions in the oligotrophic open ocean

Abstract: The phytoplankton in warm oligotrophic regions of the open oceans is dominated by ,2-mm cells too small for efficient direct consumption by mesozooplankton. However, these primary producers are hypothetically linked to higher trophic levels via the cascading impacts of mesozooplankton grazing on intermediate consumers. To assess the magnitudes of these indirect trophic linkages, grazing experiments, involving different concentration treatments of the mixed mesozooplankton community, were performed during cruises in the subtropical North Pacific at station ALOHA. Mesozooplankton fed on a diverse assemblage of microzooplankton and nanoheterotrophs .5 mm, and their predation indirectly enhanced net growth rates of phytoplankton and 2‐5-mm heterotrophs. Increasing the concentration of mesozooplankton also enhanced growth rates of heterotrophic bacteria, but this was more likely the result of organic enrichment than trophic transfer. Scaled to their natural abundance, the indirect grazing impacts of mesozooplankton on lower trophic levels are small, accounting for ,0.005 d of the growth rates of each prey category examined. Thus, the larger consumers appear to exert little net influence on the dynamics at the base of the food web. In contrast, size-fraction manipulations of consumers between 2 and 20 mm (i.e., the nanozooplankton) elicited strong responses among bacterial populations indicative of tightly coupled predatory chain of at least two steps. Given the present results, detailed studies of the interactions among pico- and nanoplankton appear to be the most profitable avenue for improving our understanding of community structure and function in this region and for acquiring useful data for developing and validating ecosystem models of the open oceans.

Algal Blooms on Coral Reefs: What Are the Causes?

Abstract: [Extract] The recent paper by Lapointe (1997) on the causes of algal blooms on coral reefs was presented as a test of the hypothesis that the reefs of Jamaica and southeast Florida had exceeded a threshold level of eutrophication that resulted in blooms of benthic algae. We offer this critique to evaluate the logic and efficacy of the tests presented by Lapointe (1997).

Atmospheric input of nitrogen and phosphorus to the Southeast Mediterranean: Sources, fluxes, and possible impact

Abstract: Estimates of the sources and wet deposition fluxes of inorganic nutrients (PO43-, NO3-, NO2-, NH4+) have been made using a long-term wet atmospheric deposition measurement at three sites along the Mediterranean coast of Israel. The nutrient composition in rainwater indicated a dominant anthropogenic source for NO, and NH: and a continental, natural, and anthropogenic, rock/soil source for PO43-. The calculated long-term dissolved inorganic N (IN) and inorganic P (IP) fluxes were 0.28 and 0.009 g m(-2) yr(-1) to the coastal zone and estimated as 0.24 and 0.008 g m(-2) yr(-1) to the Southeast (SE) Mediterranean, with a possible increasing pattern of the annual dissolved IN fluxes. Concentration of total and seawater leachable LP (LIP) from dust was examined on 20 Whatman 41 filters collected during 1996. The mean total IP concentration in dust was 0.13 +/- 0.11% (geomean = 0.09%), with a mean of 387 +/- 205 mu g IP per g of dust leached by seawater. LIP from dust varies between 6 and 85% (mean of 38%) of the dry total IF. Dust of desert-type (Saharan) events exhibited lower LIP solubility in seawater (similar to 25%, median) than air masses of European origin (similar to 45%, median). The calculated ratio of wet deposition to total (wet and dry) deposition here of 0.2 showed the importance of dry deposition of P in the SE Mediterranean basin compared to atmospheric inputs into the northwestern basin. The total IP and seawater LIP fluxes from dry deposition were estimated as 0.04 and 0.01 g m(-2) yr(-1), respectively. Atmospheric inputs of bioavailable N and P represent an imbalanced contribution to the new production of 8-20 and 4-11%, respectively, and reinforce the unusual N: P ratios (similar to 27) and possible P limitation in the SE Mediterranean.

Diversity in planktonic communities: An experimental test of the intermediate disturbance hypothesis.

Abstract: According to Connell�s intermediate disturbance hypothesis (IDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different frequencies and intensities were imposed on natural plankton communities in controlled field experiments. These disturbances consisted of an artificial deepening of the mixed layer, leading to the dilution of epilimnetic populations and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differences in the experimental mixing depth (150 and 225% of the original epilimnion depth). Investigation focused on the effect that disturbances had on the diversity of natural phytoplankton communities. Additionally, we were interested in determining the effect of grazing by zooplankton. The results of the field experiments show for the first time the applicability of the IDH to phytoplankton within complete planktonic communities. Diversity showed a clear maximum at the intermediate disturbance interval of 6 d. Similarly, species number peaked at intermediate interval length (6-10 d).

Atmospheric inputs of dissolved organic nitrogen stimulate estuarine bacteria and phytoplankton

Abstract: Atmospheric deposition is recognized as a potentially large source of inorganic nutrients to many ecosystems. In marine systems, where nitrogen (N) is the nutrient typically limiting phytoplankton growth, rainwater is often a significant source of N. Although a considerable portion of atmospheric N deposition is in the form of organic N, only the inorganic N in rainwater has been considered by most previous studies. Laboratory experiments presented here indicate that dissolved organic nitrogen (DON) from rainwater can stimulate productivity of coastal marine bacteria and phytoplankton. A large percent of rainwater DON was potentially biologically available; 45–75% was rapidly utilized by the microorganisms. The magnitude of the response of bacterial and phytoplankton biomass to equivalent amounts of DON-N or ammonium-N was similar. However, the community composition of phytoplankton at the end of the experiment differed in treatments receiving DON and inorganic N. Diatoms and dinoflagellates accounted for >90% of the phytoplankton biomass in treatments receiving rainwater DON. In contrast, small (≪2 μm) monads accounted for >85% of the biomass in the treatments receiving ammonium. The results indicate that DON in rainwater can be an important source of N to ecosystems.

Effect of CO2 concentration on C:N:P ratio in marine phytoplankton: A species comparison

Abstract: The effect of variable concentrations of dissolved molecular carbon dioxide, [CO 2,aq], on C : N : P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient-replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO 2,aq]. Among these species, the magnitude of change in C : N : P was similar over the experimental CO2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO 2 concentration below which a further decrease in [CO2,aq] led to large changes in C : N : P ratios. Significant CO 2-related changes in elemental ratios were observed at [CO2,aq] , 10 mmol kg 21 and correlated with a CO2-dependent decrease in growth rate. At [CO2,aq] typical for ocean surface waters, variation in C : N : P was relatively small under our experimental conditions. No general pattern for CO2-related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C : N and C : P with increasing [CO2,aq] was observed, depending on the species tested. Diurnal variation in C : N and C : P, tested in Skeletonema costatum, was of a similar magnitude as CO2-related variation. In this species, the CO 2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon-rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO 2related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle.

Ultraviolet‐B radiation stimulates shikimate pathway‐dependent accumulation of mycosporine‐like amino acids in the coral Stylophora pistillata despite decreases in its population of symbiotic dinoflagellates

Abstract: Colonies of Stylophora pistillata maintained for four years in indoor aquaria in the near absence of ultraviolet radiation (UVR) contained only small amounts (,5 nmol mg 21 protein) of 10 identified mycosporine-like amino acids (MAAs, which act as UV sunscreens), the largest number reported in any organism. The concentrations of most MAAs increased linearly or exponentially when colonies were exposed to ultraviolet-A (UVA) and ultraviolet-B (UVB) for 8hd 21 in the presence of photosynthetically active radiation (PAR). Total MAA concentration reached 174 nmol mg 21 protein after 30 d, with palythine and mycosporine-2 glycine constituting more than half of the final total. UVB specifically stimulated MAA accumulation: after 15 d, MAA levels in colonies exposed to PAR alone and to PAR and UVA did not differ (7 and 5 nmol MAA mg 21 protein, respectively), while those in colonies exposed to PAR and UVA 1 UVB were significantly higher (28 nmol mg 21 protein). Glyphosate, an inhibitor of the shikimate pathway, eliminated or reduced the UV-induced accumulation of most MAAs during 7 d of exposure, providing the first experimental evidence of their synthesis via this pathway in a coral symbiosis. Densities of zooxanthellae in colonies of S. pistillata, Acropora sp., and Seriatopora hystrix exposed to UVR for 15 d were only one-third of those in control colonies unexposed to UVR. This net decrease in the number of zooxanthellae in the corals (bleaching) occurred despite UV-stimulated increases in algal cytokinesis and in the host cell-specific density of zooxanthellae in hospite, increases that apparently destabilized the symbiosis and caused expulsion of the zooxanthellae.

The importance of siderophores in iron nutrition of heterotrophic marine bacteria

Abstract: Recent studies demonstrate that dissolved iron in seawater is bound to strong organic complexes that have stability constants comparable to those of microbial iron chelates. We examined iron acquisition by seven strains of heterotrophic marine bacteria from a number of siderophore‐iron complexes, including desferrioxamine B (DFB) and marine siderophores partially purified from iron-limited cultures. Hydroxamate siderophores were detected in the supernatants of four strains, one of which also produced a catechol. All strains transported iron bound to siderophores regardless of whether or not they produced their own, and the majority took up iron bound to DFB. Uptake rates of Fe siderophores were similar among iron-limited strains and among ligands. Transport of FeDFB by strain Neptune was enhanced 20 times by iron limitation, whereas uptake of unchelated iron (Fe9) did not saturate at the highest concentration tested and was not regulated by the iron nutritional status of the cells. The half-saturation constant for uptake of FeDFB by Neptune was 15 nM, the lowest reported for an Fe siderophore in any microorganism. Iron uptake by the catechol-producing strain, LMG1, differed markedly in two respects from the other strains: LMG1 could not take up iron bound to DFB; furthermore, transport of Fe9 by iron-limited LMG1 was 10 times faster than the other strains and was upregulated 46 times compared to Fe-sufficient cells. Experimental evidence suggests that iron transport by LMG1 may be mediated by surface-associated catechol siderophores that scavenge inorganic ferric species as well as iron bound to weaker complexes, such as EDTA (ethylenediaminetetraacetic acid). The combined results of the study highlight the importance of siderophores in iron transport by heterotrophic marine bacteria and suggest, by inference, that bacteria may rely on siderophores to acquire iron in situ.

Copper-induced release of complexing ligands similar to thiols by Emiliania huxleyi in seawater cultures

Abstract: Marine microalgae (Emiliania huxleyi) were grown in seawater enriched only with nitrogen and phosphorus, without control of free metal concentrations using synthetic chelators. Complexing ligands and thiol compounds were determined by cathodic stripping volt-ammetry. Copper was added to these cultures, and ligands were produced in response to the copper addition. Parallel measurements of thiols showed that glutathione and other unidentified thiols (electrochemically similar to thioacetamide) were produced by the algae at rates and concentrations similar to those of the complexing ligands. Smaller amounts of thiols were produced when ligands including thiols were added to the culture. The results indicate that thiols can account for a major part or most of the copper-complexing ligands produced by these algae. Furthermore, a feedback mechanism exists in which the production of thiol-type complexing ligands is controlled by the free copper concentration, production already being stimulated by an increase of [Cu 21 ] from 0.4 to 1.5 pM. Incubations with added exudates, thiols, and salicylaldoxime (SA) showed much reduced copper toxicity even though copper uptake was increased by the exudates and the SA.

Inorganic nitrogen acquisition in the seagrass Thalassia testudinum: Development of a whole‐plant nitrogen budget

Abstract: Whole-plant nitrogen (N) uptake experiments were used to quantify the N budget of Thalassia testudinumgrowing under different sediment nutrient regimes at two locations in the western Gulf of Mexico. At both sites, Corpus Christi Bay (CCB) and lower Laguna Madre (LLM), Texas, concurrent measurements of plant biomass and levels of dissolved inorganic nitrogen (DIN) in the water column and sediments were made over a 12-month period (October 1996‐October 1997). Water-column NH and NO 1 NO concentrations were not significantly different 12 2 43 2 between study sites (ca. 1.2 mM [NH ] and 0.7 mM [NO 1 NO ] at both sites), but sediment NH concentrations 12 2 1 43 2 4 in CCB (87 mM) were significantly higher than in LLM (26mM). The higher sediment NH levels at CCB correlated 1 4 with significantly higher leaf biomass at CCB, but there was no difference in root biomass between study sites. Leaf NH uptake showed clear seasonal variation: Vmax was highest in summer and fall, but Km was highest in 1 4 winter. Vmax of leaf NO uptake did not change with season, but Km decreased with increasing incubation temperature. 2 3 There were no clear differences in leaf NH and NO uptake rates between study sites, although leaf NH uptake 12 1 43 4 affinity was higher than that of NO . Root NH uptake was variable with season and did not saturate at the 21 34 experimental NH concentrations at either site (0‐300 mM). Based on these measurements, N acquisition was 1 4 highest during summer and fall and lowest during winter and spring. Roots and leaves contributed nearly equally to total plant N acquisition (root NH 5 52%; leaf NH 5 38%; and leaf NO 5 10%) at both sites. Annual N 11 2 44 3 acquisition in CCB was double that of LLM (97.03 and 53.49 g N m 22 yr 21 , respectively), but .50% of N uptake was not incorporated into biomass at either site. DIN turnover time ranged from 0.21 to 0.91 d in the water column and from 0.95 to 1.75 d in sediment pore water, indicating the importance of DIN regeneration processes for supporting seagrass production. The similarity in the relative tissue contributions between plants at both sites, despite a significant difference in sediment NH pool sizes, results from the higher fraction of biomass allocated to below1 4 ground tissues in plants living under low-sediment N conditions (LLM). In N-sufficient sediments, overall plant productivity is greater as T. testudinum is able to allocate a greater proportion of its biomass into photosynthetic aboveground tissues.