Showing papers in "Limnology and Oceanography in 2000"

Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton

Abstract: Cellular carbon and nitrogen content and cell volume of nutritionally and morphologically diverse dinoflagellate species were measured to determine carbon to volume (C : vol) and nitrogen to volume (N : vol) relationships. Cellular C and N content ranged from 48 to 3.0 3 10 4 pgC cell 21 21 5

Elements of a theory for the mechanisms controlling abundance, diversity, and biogeochemical role of lytic bacterial viruses in aquatic systems

Abstract: Mechanisms controlling virus abundance and partitioning of loss of bacterial production between viral lysis and protozoan predation are discussed within the framework of an idealized Lotka-Volterra‐type model. This combines nonselective protozoan predation with host-selective viral lysis of bacteria. The analysis leads to a reciprocal relationship between bacterial diversity and viruses, in which coexistence of competing bacterial species is ensured by the presence of viruses that ‘‘kill the winner,’’ whereas the differences in substrate affinity between the coexisting bacterial species determine viral abundance. The ability of the model to reproduce published observations, such as an approximate 1 : 10 ratio between bacterial and viral abundance, and the ability of viral lysis to account for 10‐ 50% of bacterial loss are discussed.

Total nitrogen, total phosphorus, and nutrient limitation in lakes and oceans: Is there a common relationship?

Abstract: ~~~~~~~~~~~~~Number 6 Abstract Total nitrogen (TN) and total phosphorus (TP) measurements and contemporaneous measurements of chlorophyll a (Chl a) and phytoplankton nutrient deficiency have been made across a broad range of lakes and ocean sites using common methods. The ocean environment was nutrient rich in terms of TN and TP when compared with most lakes in the study, although Lake Victoria had the highest values of TN and TP. TN concentrations in lakes rose rapidly with TP concentrations, from low values to TN concentrations that are similar to those associated with the ocean sites. In contrast, the TN concentrations in the oceans were relatively homogeneous and independent of TP concentrations. The hyperbolic shape of the TN: TP relationship created a broad range of TN: TP values for both lakes and oceans. The TN: TP ratios of the surface ocean sites were usually well in excess of the Redfield ratio that is noted in the deep ocean. Phytoplankton biomass, as indicated by Chl a, was strongly dependent upon TP in the lakes, and there was a weaker relationship with TN. Oceanic Chl a values showed a positive relationship with TP, but at much higher TP values than were observed in the lakes; there was no relation with TN. P-deficient phytoplankton growth was inferred using independent indicators when TP was 50 (molar). At intermediate TN TP ratios, either N or P can become deficient. We conclude that N or P limitation of algal growth is a product of the TN and TP concentration and the TN: TP ratio rather than a product of whether the system of study is marine or freshwater.

Carbon loss and optical property changes during long‐term photochemical and biological degradation of estuarine dissolved organic matter

Abstract: Terrestrially derived dissolved organic matter (DOM) impacts the optical properties of coastal seawater and affects carbon cycling on a global scale. We studied sequential long-term photochemical and biological degradation of estuarine dissolved organic matter from the Satilla River, an estuary in the southeastern United States that is dominated by vascular plant-derived organic matter. During photodegradation, dissolved organic carbon (DOC) loss (amounting to 31% of the initial DOC) was much less extensive than colored dissolved organic matter (CDOM) or fluorescent dissolved organic matter (FDOM) loss (50% and 56% of the initial CDOM and FDOM), and analysis of kinetics suggested a reservoir of DOC that was resistant to photodegradation. In contrast, CDOM photodegradation closely followed first-order kinetics over two half-lives with no indication of a nondegradable component. FDOM loss was slightly biased toward fluorophores considered representative of terrestrial humic substances. Additional changes in optical properties included increases in spectral slope and shifts in fluorescence excitation/ emission maxima that were generally consistent with previous observations from field studies of photobleached DOM. Biological degradation of photobleached DOM was more rapid than that of unbleached material, and this net positive effect was evident even for extensively photodegraded material. Bacterial degradation caused shifts in the opposite direction from photochemical degradation for both spectral slope and excitation/emission maxima and thus dampened but did not eliminate changes in optical properties caused by photobleaching.

The fate of intertidal microphytobenthos carbon: An in situ 13C-labeling study

Abstract: At two intertidal sites (one sandy and one silty, in the Scheldt estuary, The Netherlands), the fate of microphytobenthos was studied through an in situ C-13 pulse- chase experiment. Label was added at the beginning of low tide, and uptake of C-13 by algae was linear during the whole period of tidal exposure (about 27 mg m(-2) h(-1) in the top millimeter at both sites). The C-13 fixed by microphytobenthos was rapidly displaced toward deeper sediment layers (down to 6 cm), in particular at the dynamic, sandy site. The residence times of microphytobenthos with respect to external losses (resuspension and respiration) were about 2.4 and 5.6 d at the sandy and silly stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of C-13 in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of C-13 into bacterial biomass was quantified by carbon isotope analysis of polar lipid derived fatty acids specific for bacteria. The bacterial polar lipid-derived fatty acids (i14:0, i15:0, a15:0, i16:0, and 18:1 omega 7c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and C-13 assimilation increased until day 3. Microphytobenthos carbon entered all heterotrophic components in proportion to heterotrophic biomass distribution (bacteria > macrofauna > meiofauna). Our results indicate a central role for microphytobenthos in moderating carbon flow in coastal sediments. [KEYWORDS: Water marine habitats; microbenthic communities; westerschelde estuary; microbial biomass; epipelic diatoms; ecological role; organic-matter; grazing rates; chlorophyll-a; secret garden]

Nutrient and temperature control of the contribution of picoplankton to phytoplankton biomass and production

Abstract: The observation that the relative importance of picophytoplankton is greatest in warm and nutrient-poor waters was tested here based on a comprehensive review of the data available in the literature from oceanic and coastal estuarine areas. Results show that picophytoplankton dominate ($50%) the biomass and production in oligotrophic (chlorophyll a [Chl a] , 0.3 mg m 23 ), nutrient poor (NO3 1 NO2 , 1 mM), and warm (.268C) waters, but represent ,10% of autotrophic biomass and production in rich (Chl a . 5m g m 23 ) and cold (,38C) waters. There is, however, a strong covariation between temperature and nutrient concentration (r 52 0.95, P , 0.001), but the number of observations where both temperature and nutrient concentrations are available is too small to allow attempts to statistically separate their effects. The results of mesocosm nutrient addition experiments during summer in the Mediterranean Sea allowed the dissociation of the effects of temperature from those of nutrients on picophytoplankton production and biomass and validated the magnitude at which picoplankton dominates ($50%) autotrophic biomass and production obtained in the comparative analysis. The fraction contributed by picoplankton significantly declined (r 2 5 0.76 and 0.90, respectively, P , 0.001) as total autotrophic production and biomass increased. These results support the increasing importance of picophytoplankton in warm, oligotrophic waters. The reduced contribution of picophytoplankton in warm productive waters is hypothesized here to be due to increased loss rates, whereas the dominance of picophytoplankton in warm, oligotrophic waters is attributable to the differential capacity to use nutrients as a function of differences in size and capacity of intrinsic growth of picophytoplankton and larger phytoplankton cells.

Seasonal patterns of tissue biomass and densities of symbiotic dinoflagellates in reef corals and relation to coral bleaching

Abstract: Tissue biomass (ash-free dry weight) and symbiotic dinoflagellates (density, chlorophyll a cell 21 or cm 22 of coral surface area) of five species of reef-building corals were monitored seasonally for up to 4 yr at three different depths in the Bahamas. The lowest values of all tissue biomass and algal symbiont parameters occurred during the late summer‐fall sample periods. In contrast, the highest densities and pigment content of symbionts usually occurred during the winter, whereas tissue biomass peaked most often in the spring, the time lag implying a functional relationship between these variables. Corals living in shallow water often (but not always) had higher levels of all parameters measured compared to deeper corals, except chlorophyll a content, which usually displayed the opposite trend. The results show that corals from all depths exhibited bleaching (loss of symbiotic dinoflagellates and/or their pigments) every year, regardless of whether they appeared white, tan, or mottled to the human eye. We speculate that these patterns are driven by seasonal changes in light and temperature on algal and animal physiology. Furthermore, we hypothesize that all tropical reef-building corals, world-wide, exhibit similar predictable cycles in their tissue biomass and symbiotic algae.

Persistence of net heterotrophy in lakes during nutrient addition and food web manipulations

Abstract: Net ecosystem production (NEP) is the difference between gross primary production (GPP) and community respiration (R). We estimated in situ NEP using three independent approaches (net CO2 gas flux, net O 2 gas flux, and continuous diel O2 measurements) over a 4‐7 yr period in a series of small lakes in which food webs were manipulated and nutrient loadings were experimentally varied. In the absence of manipulation, these lakes were net heterotrophic according to all three approaches. NEP (NEP 5 GPP-R) was consistently negative and averaged 235.5 6 3.7 (standard error) mmol C m 22 d 21 . Nutrient enrichment, in the absence of strong planktivory, tended to cause increases in estimates of both GPP and R (estimated from the continuous O2 data) but resulted in little change in the GPP/R ratio, which remained ,1, or NEP, which remained negative. When planktivorous fish dominated the food web, large zooplankton were rare and nutrient enrichment produced positive values of NEP by all three methods. Among lakes and years, daily values of NEP ranged from 2241 to 1175 mmol m 22 d 21 ; mean seasonal NEP was positive only under a combination of high nutrient loading and a planktivore-dominated food web. Community R is significantly subsidized by allochthonous sources of organic matter in these lakes. Combining all lakes and years, we estimate that ;26 mmol C m 22 d 21 of allochthonous origin is respired on average. This respiration of allochthonous organic matter represents 13 to 43% of total R, and this fraction declines with increasing GPP.

The effect of dietary nitrogen content on trophic level 15N enrichment

Abstract: Given the great potential value of stable isotopes in a variety of scientific investigations, surprisingly little attention has been paid to the underlying physiological and biochemical mechanisms that account for trophic increases in d 15 N values. This has lead to a general call for controlled studies investigating the relationship between organismal diet and corresponding isotopic composition. We conducted a series of laboratory studies varying dietary nitrogen content and measuring corresponding variations in organismal d 15 N values. Specifically, we investigated the relationship between the d 15 N values of the anomopod crustacean, Daphnia magna, and the C : N ratio of its food, the green algae, Scenedesmus acutus.Daphnids were raised to a standard life stage on three types of S. acutus as food, which ranged in C : N (atomic) from 7.3 to 24.8. The average C : N of the daphnids was 6.0. 15 N enrichment was found to be strongly linearly related to the C : N of the algae, ranging from nearly zero to approximately 6‰, which would normally be considered a span of almost two trophic levels. The d 15

Modeling basin-scale internal waves in a stratified lake

Abstract: Basin-scale internal waves provide the driving forces for vertical and horizontal fluxes in a stratified lake below the wind-mixed layer. Thus, correct modeling of lake mixing and transport requires accurate modeling of basin- scale internal waves: examining this capability with a hydrostatic, z-coordinate three-dimensional (3D) numerical model at coarse grid resolutions is the focus of this paper. It is demonstrated that capturing the correct thermocline forcing with a 3D mixed-layer model for surface dynamics results in a good representation of low-frequency internal wave dynamics. The 3D estuary and lake computer model ELCOM is applied to modeling Lake Kinneret, Israel, and is compared with field data under summer stratification conditions to identify and illustrate the spatial structure of the lowest-mode basin-scale Kelvin and Poincarewaves that provide the largest two peaks in the internal wave energy spectra. The model solves the unsteady Reynolds-averaged Navier-Stokes equations using a semi-implicit method similar to the momentum solution in the TRIM code with the addition of quadratic Euler-Lagrange dis- cretization, scalar (e.g., temperature) transport using a conservative flux-limited approach, and elimination of vertical diffusion terms in the governing equations. A detailed description is provided of turbulence closure for the vertical Reynolds stress terms and vertical turbulent transport using a 3D mixed-layer model parameterized on wind and shear energy fluxes instead of the convential eddy viscosity/diffusivity assumption. This approach gives a good representation of the depth of the mixed-layer at coarse vertical grid resolutions that allows the internal waves to be energized correctly at the basin scale. Wind stresses, surface heating, and density currents form the driving energy fluxes of a stratified lake. The basin-scale energy flux from the wind is of particular interest because of its dominant role in setting the thermocline in motion, which, in the absence of inflows and outflows, is the primary energy store for transport and mixing below the wind-mixed layer. Thus, modeling the basin-scale internal wave behavior is an a priori requirement to modeling and quantifying the flux paths of nutrients in a stratified lake (Imberger 1994). This paper takes a first step in this direction by analyzing our ability to model basin-scale internal waves that are seen in Lake Kinneret, Israel. Energy flux path in a stratified lake —Energy flux through a stratified lake has a fundamental dependence on forced and free baroclinic motions. The wind imparts both momentum and turbulent kinetic energy (TKE) to the water in the sur- face layer. The TKE distributes momentum vertically in the

Photochemical transformation of dissolved organic matter in lakes

Abstract: In a survey of photochemical transformation of dissolved organic matter (DOM) in lake water, we found photochemical production of dissolved inorganic carbon (DIC) and low molecular weight carboxyli ...

Cyanobacterial blooms in the Baltic Sea: Natural or human-induced?

Abstract: Massive summer blooms of nitrogen-fixing cyanobacteria have been documented in the Baltic Sea since the 19th century, but are reported to have increased in frequency, biomass, and duration in recen

Transport and degradation of phytoplankton in permeable sediment

Abstract: In flume and field experiments we demonstrate that interfacial water flows, generated when bottom currents interact with sea bed topography, provide a fast and efficient pathway for the transport of suspended phytoplankton into subsurface layers of permeable sandy sediments. The advective transport, associated with small mounds and ripples as commonly found on shelf sediments, increased penetration depth of unicellular algae (Dunaliella spec.) into sandy sediment (permeability k 5 4 3 10 211 m 2 ) up to a factor of 7 and flux up to a factor of 9 relative to a smooth control sediment. The pore water flow field produced a distinct distribution pattern of particulate organic matter in the sediment with subsurface concentration maxima and zones depleted of algae. Flux chamber simulations of advective transport of algae into sands of different grain sizes revealed increasing fluxes, algal penetration depths, and degradation rates with increasing permeability of the sediment. Two experiments conducted in intertidal sand flats confirmed the importance of the advective interfacial transport of phytoplankton for natural settings, showing permeability-dependent penetration of planktonic algae into embedded sand cores of different grain sizes. The significance of our results is discussed with respect to particulate organic matter flux and mineralization in shelf sands, and we suggest the concept of a decomposition layer. In contrast to muddy sea beds with low permeabilities, where transport of solutes is mainly driven by diffusion, water can flow through marine sands, providing a fast carrier for the exchange of substances between the water column and the upper sediment layers. Surface gravity waves cause pressure oscillations that increase fluid exchange at the sediment‐water interface and dispersion of solutes within the

Accumulation of heavy metals in food web components across a gradient of lakes

Abstract: Recent studies have emphasized the need for understanding the accumulation and fate of metal contaminants at different trophic levels and across a broad spectrum of lake types. To address both issues, metal concentrations (Hg, Zn, Cd, As, and Pb) were measured in the water, two size fractions of zooplankton, and fish from 20 lakes in contaminated to pristine watersheds in the northeastern United States. Our goals were to examine links between watershed characteristics and aqueous metal levels in lakes and relationships between aqueous concentrations, metal burdens in different plankton groups and in fish. Two pairs of metals, (1) Hg and Zn and (2) As and Pb, exhibited strong similarities both in the factors that predict their concentrations in water and in the patterns of accumulation in particular trophic levels. Aqueous concentrations of Hg and Zn were highest in cool water lakes, whereas As and Pb were highest in more eutrophic lakes in agricultural areas. Aqueous Cd concentrations were closely correlated with the land-use variables, percentage of agricultural land, and road densities. Similarly, Hg and Zn both biomagnified from small plankton (45‐202mm) to macrozooplankton (.202 mm) and from macrozooplankton to fish. In contrast, bioaccumulation of both As and Pb diminished with increasing trophic level. Although aqueous metal and zooplankton metal levels were not significant predictors of As and Pb levels in fish, metal levels in zooplankton were predictive of Hg and Zn in fish, suggesting that sources of bioaccumulation differ for different metals. Our findings demonstrate the importance of investigating upper and lower trophic levels separately, to fully understand metal transfer pathways in aquatic food webs.

Climate-driven changes in spring plankton dynamics and the sensitivity of shallow polymictic lakes to the North Atlantic Oscillation

Abstract: Climate is increasingly recognized as a major factor driving long-term changes in plankton communities in both marine and limnetic ecosystems. In a shallow and polymictic lake, marked changes in the dynamics of phytoplankton and in the timing of distinctive successional events in spring were observed during two recent decades. We studied whether and how these changes were linked to a series of uncommonly warm winter and spring seasons, focusing on the predictive power of a macroscale atmospheric circulation pattern, the North Atlantic Oscillation (NAO). In the warm period 1988‐1998, phytoplankton developed about 1 month earlier than in the cool period 1979‐1987, and high total phytoplankton biomass was recorded in early spring. These changes were significantly related to the NAO, which explained about 35% of the variance, and can be attributed to a shortening or lack of ice cover periods in winter. Zooplankton (Keratella, Bosmina, Daphnia ) developed about 2 weeks earlier in the warm period, consequently leading to an early establishment of the clearwater phase. The shift in the timing of the daphnid peak and of the clearwater phase required an additional warming trend in late April and early May and was not significantly correlated with the winter situation. The memory effect of the NAO was restricted to a rather short postwinter period and was overtaken by the prevailing weather in April. Our results suggest that assessments of potential impacts of climate warming on biological processes in freshwater ecosystems demand basic knowledge about the relationships between global climate indices, local meteorological conditions, and the thermal response of various lake types.

Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary

Abstract: Direct measurements of the partial pressure of CO2 (pCO2) and dissolved inorganic carbon (DIC) were made over a 2-yr period in surface waters of the York River estuary in Virginia. The pCO2 in surface waters exceeded that in the overlying atmosphere, indicating that the estuary was a net source of CO 2 to the atmosphere at most times and locations. Salinity-based DIC mixing curves indicate there was also an internal source of both DIC and alkalinity, implying net alkalinity generation within the estuary. The DIC and alkalinity source displayed seasonal patterns similar to that of pCO2 and were reproducible over a 2-yr study period. We propose that the source of inorganic carbon necessary for both the sustained CO2 evasion to the atmosphere and the advective export of DIC is respiration in excess of primary production (e.g., net heterotrophy). The rates of CO2 evasion and DIC export were estimated to provide an annual rate of net heterotrophy of ; 100 gCm 22 yr 21 . Approximately 40% of this excess inorganic carbon production was exported as DIC to the coastal ocean, whereas 60% was lost as CO2 evasion to the atmosphere. The alkalinity generation needed to sustain the export of inorganic carbon, as HCO3 2 , is most likely provided by net sulfate reduction in sediments. Accumulation of sulfide in the sediments of a representative site directly adjacent to the York River estuary is sufficient to account for the net export of alkalinity. The seasonality of net heterotrophy causes large variations in annual CO 2 and DIC concentrations, and it stresses the need for comprehensive temporal data sets when reporting annual rates of CO 2 evasion, DIC advection, and net heterotrophy.

Attenuation of ultraviolet radiation in mountain lakes: Factors controlling the among‐ and within‐lake variability

Abstract: High-altitude lakes are exposed to high fluence rates of solar ultraviolet radiation (UVR; 290‐400 nm) and contain low concentrations of dissolved organic carbon (DOC) While in most lowland lakes, DOC can be used to predict UV transparency with sufficient accuracy, current models fail to estimate UVR in clear alpine lakes In these lakes, phytoplankton may contribute significantly to the UV attenuation either as particles or as a source of chromophoric dissolved organic matter (CDOM) with distinctive properties We investigated a series of 26 lakes in the Alps and Pyrenees, situated at elevations ranging from 422 to 2,799 m above sea level and having DOC concentrations ranging from 02 to 35 mg L 21 CDOM, as measured by the absorptivity of filtered lake water, explained most of the variability in the attenuation of underwater UVR among lakes (r 2 5 094, P , 0001) However, within-lake variation in the UV attenuation revealed a significant contribution from phytoplankton in deeper waters (UV attenuation increasing with chlorophyll a concentration; r 2 5 097, P 5 0002), only apparent when DOC concentrations were low (;03 mg L 21 ) The DOC-specific absorptivity (ag*) was also important for characterizing the optical conditions in this series of lakes Epilimnetic values of ag* were significantly lower in lakes located at high elevations (with low allochthonous CDOM inputs from the catchment), compared to lakes surrounded by trees and meadows Moreover, ag* was generally lower in surface waters than in deeper water layers, suggesting the influence of photobleaching on UV transparency The slope S of the exponential regression between CDOM absorptivity and wavelength did not show clear patterns, such as found in marine systems, and often presented lower values in the epilimnetic waters (in association with lower ag*) Collectively, our results suggest that in transparent alpine lakes, the dynamics of the CDOM pool and phytoplankton production will have a strong effect on temporal changes in UV underwater attenuation

Regulation of nitrification in aquatic sediments by organic carbon

Abstract: Nitrification, the microbial conversion of ammonium to nitrate, is an important transformation in the aquatic nitrogen cycle, but the factors regulating nitrification rates in freshwater ecosystems are poorly understood. We investigated the effects of organic carbon quantity and quality on nitrification rates in stream sediments. First, we hypothesized that when environmental C: N ratios are high, heterotrophic bacteria are subject to N limitation and will outcompete nitrifying bacteria for available NH4+, thereby reducing nitrification rates. In laboratory experiments, organic carbon amendments (30 mg C L−1, as glucose) to stream sediments completely inhibited nitrification with or without addition of NH4+ (P < 0.0001), whereas amendment with NH4+ only (0.75 mg N L−1) increased nitrification by 40% compared with unamended controls (P < 0.0001). Carbon amendments also increased microbial respiration rates over controls by 4–6 times. Therefore, organic carbon additions significantly decreased nitrification rates but increased total microbial activity. Second, we hypothesized that carbon of high quality would have a stronger negative effect on nitrification than would carbon of low quality. To stream sediments, we added organic carbon as either glucose (higher quality) or sugar maple leaf extract (lower quality). Nitrification rates were reduced by the addition of either organic carbon source but were more severely inhibited by glucose (P = 0.001). Our results suggest that organic carbon is an important regulator of nitrification rates and is of key importance in understanding N dynamics in freshwater ecosystems.

Effects of nutrient enrichment and small predator density on seagrass ecosystems: An experimental assessment

Abstract: We used a field experiment to assess the individual and combined effects of removing top predators and enriching water column nutrients (nitrogen-N and phosphorus-P) on seagrass ecosystem structure and function. Experiments were conducted in turtlegrass (Thalassia testudinum) habitats in St. Joseph Bay, FL, an aquatic preserve in the northern Gulf of Mexico that exhibits low ambient nutrient concentrations and contains abundant populations of small crustacean and gastropod mesograzers. We stocked 7.0 m 2 enclosures with elevated (;4‐83 ambient) densities of juvenile pinfish (Lagodon rhomboides), the dominant fish species in local seagrass habitats, to simulate the first-order effects of large predator reductions, and we used an in situ delivery system to supplement N and P to ;33 ambient levels in nutrient addition treatments. Monthly determinations of water column nutrients and Chl a, along with measurements of the biomass and abundance of leaf epiphytes and seagrass production, biomass, and shoot and leaf densities were used to evaluate the relative effects of manipulating nutrient supply and altering food web structure. In contrast to our expectations, results showed few significant nutrient effects, or fish 3 nutrient enrichment effects on any of the parameters measured. However, there were many significant fish effects, most of which were unexpected. As predicted, increased pinfish density reduced mesograzer numbers significantly. Not anticipated, however, was the reduced epiphyte biomass in fish enclosure treatments, apparently brought about by the pinfish consuming significant amounts of epiphytes as well as mesograzers. This reduction in epiphyte biomass produced positive indirect effects on seagrass biomass, shoot number, and rates of primary productivity in pinfish enclosure treatments. Our results also showed important top-down effects in determining the composition and abundance of seagrassassociated plants and animals in this pristine environment. Although we did not observe simple trophic cascades, most likely because pinfish fed at more than one trophic level, and because the dense seagrass prevented small grazers from being reduced to low numbers, pinfish produced important changes in the epibiota as well as the seagrasses themselves. These data, while contrasting with studies reporting significant negative nutrient enrichment effects on seagrasses, support the results of recent experimental studies in showing that: (1) small grazers can often control the abundance of epiphytes; and (2) it is unlikely that a full understanding of the consequences of nutrient enrichment for seagrass ecosystems can be gained without knowing how grazer population are regulated.

The record of nitrate utilization and productivity limitation provided by δ15N values in lake organic matter—A study of sediment trap and core sediments from Baldeggersee, Switzerland

Abstract: The response of nitrogen isotopic composition of organic matter to changes in nitrate utilization, N-limitation and lake productivity was studied in Baldeggersee, a small eutrophic lake in central Switzerland. Nitrogen isotope ratios were measured on organic matter accumulation in sediment traps, collected daily from March 1995 through October 1996, and on individually sampled light and dark annual laminae from a 108-yr sediment core sequence (1885‐1993). Nitrogen accumulation in the sediment traps averaged 0.04 g N m 22 d 21 . d 15 N values of the sediment trap material increased from 11‰ to 13‰ (atmospheric N 2) as primary productivity decreased surface water [NO3 2 ] from 1.4 to 0.7 mg N L 21 during the seasonal stratified periods. Very small amounts of isotopically enriched organic matter (d 15 N ; 15‐20‰) of heterotrophic and/or detrital origin accumulated in the winter months. Nitrogen accumulation in the core sediments average 4 g N m 22 per year. d 15 N values of the sediment core material increased up-core (from d 15 N ; 6‰ to d 15 N ; 11‰); several abrupt positive isotope shifts ( .2‰) occur in the upper part of the record. Comparison between sediment d 15 N values and surface water [NO3 2 ] measurements for the period of 1976‐1993 reveals that the abrupt positive d 15 N shifts occurred in years where unusually large phytoplankton blooms depleted surface waters nitrate to concentrations of ,0.7 mg N L 21 .A 3‰ negative d 15 N shift, observed between 1973‐1975 at the time of maximum anoxic conditions and meromixis, can be attributed to phytoplankton incorporation of ammonia, which was present in concentrations of up to 0.7 mg L 21 in the epilimnion. Preservation of isotopic shifts in the sediment core which are clearly related to water column processes, and the similarity of core top d 15

Abundance and diversity of prymnesiophytes in the picoplankton coμmunity from the equatorial Pacific Ocean inferred from 18S rDNA sequences

Abstract: Picoplankton, i.e., cells smaller than 2‐3 mm, dominate in most open oceanic regions, such as in the Pacific Ocean. In these areas, the dominant carotenoid of photosynthetic eukaryotes is 199-hexanoyloxyfucoxanthin (19HF), considered to be a diagnostic marker for prymnesiophytes. This suggests that this class could be a major component of eukaryotic picoplankton, despite the fact that virtually no prymnesiophyte has been described to date from this size class. To address this question, we assessed prymnesiophyte diversity and abundance in natural picoplankton communities, using a molecular approach. Total genomic DNA was isolated from 3-mm-filtered samples collected in the Pacific Ocean. Small subunit (18S) ribosomal RNA genes (rDNA) were amplified by the polymerase chain reaction (PCR) using universal eukaryotic primers. The relative abundance of 18S rDNA from prymnesiophytes was quantified using group-specific and eukaryotic 18S rDNA probes. The percentage of the prymnesiophyte versus total 18S rDNA was much lower than the percentage of prymnesiophytes calculated on the basis of pigment analyses for the same samples. 18S rDNA libraries from five samples were screened using a prymnesiophyte-specific oligonucleotide probe, and 14 nearly complete 18S rDNA sequences were retrieved. Phylogenetic analysis of these sequences established the presence of several prymnesiophyte lineages with no equivalent among cultivated species. It is now well established that cells smaller than 2‐3 mm (picoplankton), rather than the larger microalgae, dominate the phytoplankton community in the open ocean. In particular, the eukaryotic component of picoplankton, the so-called picoeukaryotes, has been recognized to contribute significantly to both primary biomass and production in open oceanic regions (Campbell et al. 1994; Li 1994). Its taxonomic 1

Organic matter in Bolivian tributaries of the Amazon River: A comparison to the lower mainstream

Abstract: We determined the concentrations and compositions of coarse particulate (.63 mm), fine particulate (0.1‐63mm), and dissolved (0.001‐0.1 mm) organic matter collected along a river reach extending from a first-order stream in the Bolivian Andes, through the Beni River system, to the lower Madeira and Amazon Rivers. Dissolved organic carbon (DOC) concentrations increased down the total reach from ;80 to 350 mM. The percentage of total DOC with a molecular weight greater than ;1,000 atomic mass units that could be isolated by ultrafiltration also increased downstream from 40 to 80%. Weight percentages of organic carbon in the ultrafiltered isolates also grew downstream from 5% at the uppermost station to 37% in the Amazon mainstem. Organic carbon composed only 0.4‐1.2 weight percentage of the total mass of the fine particulate fraction, which accounted for 70‐80% of the total organic carbon (TOC) in transport through the highly turbid (;600‐2000 mg L 21

Effect of Zn, Mn, and Fe on Cd accumulation in phytoplankton: Implications for oceanic Cd cycling

Abstract: Cd and phosphate concentrations in seawater are closely related, suggesting that Cd distributions, like those of PO4, are controlled by algal uptake and regeneration. But the factors that control Cd levels in phytoplankton and their influence on oceanic Cd versus PO 4 relationships are poorly known. We examined the effect of important controlling factors (free ion concentrations of Cd, Zn, and Mn and Fe limitation of growth rate) on Cd accumulation by an oceanic diatom (Thalassiosira oceanica). More limited comparative experiments were also conducted with a coastal diatom (T. weissflogii) and an oceanic coccolithophore (Emiliania huxleyi). Cellular Cd : C ratios increased with increasing Cd ion concentrations and decreasing Zn ion concentrations in all species and with decreasing ionic Mn in the diatoms. The effects of Mn and Zn apparently are related to the uptake of Cd by the cells’ Mn and Cd/ Co transport systems, which are under negative feedback regulation by cellular Mn and Zn. Cd : C ratios were determined in the oceanic diatom as functions of free ion concentrations of Cd, Zn, and Mn over the oceanic range of these metal ions. The data were combined with estimates of free ion concentrations of these metals in seawater to construct models of Cd : C and Cd : P ratios in oceanic phytoplankton. The modeled Cd : P ratios showed good agreement with slopes of Cd versus P relationships in different oceanic regimes. The models suggest that the high Cd versus phosphate slopes found in iron-depleted waters of the Southern Ocean and subarctic Pacific result from unusually high levels of zinc depletion in these waters. The resulting low zinc ion concentrations induce high levels of Cd uptake by phytoplankton, yielding high algal Cd : P ratios. The heavy depletion of zinc may be linked to elevated Zn : C and Zn : P ratios in iron-limited diatoms, as observed in experiments we conducted with T. oceanica.

Interannual variation in the thermal structure of clear and colored lakes

Abstract: We used end-of-summer temperature profiles to examine the thermal structure of 86 small (500 ha) lakes in Killarney Park, Ontario, Canada, during one cool (1997) and two extremely warm years (1998 and 1999). The main effect of the warm years, which had unusually high air temperatures during the spring, relative to the cool year was to create warmer surface waters, shallower mixing depths, and stronger metalimnetic thermal gradients in nearly all lakes. Changes in deep water temperatures differed between clear (DOC , 2m g L 21 ) and colored (DOC . 4 21 ) lakes. During warm years, the volume of cold water (,108C) was reduced in clear lakes. In colored lakes, deep water temperatures were more stable, and cold water volume actually increased during one warm year. We suggest that clear lakes will be more sensitive than colored lakes to the warming effects of climate change. Because clear lakes exhibit large thermal changes in response to small differences in DOC, they will also be more sensitive to changes in DOC levels associated with altered hydrological inputs, climate change, or acidification.

Latitudinal patterns in the size distribution and seasonal dynamics of new world, freshwater cladocerans

Abstract: Analysis of zooplankton communities in over 1,100 water bodies spanning the Western Hemisphere (778S, 818N latitude) revealed latitudinal patterns in cladoceran body size and Daphnia seasonal dynamics. The mean body length of cladoceran species occurring at different latitudinal intervals was greatest in north temperate regions, declining in size toward the poles and equator. Thus, mean cladoceran size was greatest in regions where the mean annual surface temperatures of lakes ranged from 6 to 88C in both the Northern and Southern Hemispheres. The date of maximum Daphnia abundance and the period of minimum Daphnia abundance in lakes of North America was positively, linearly related to the latitudinal position of the lakes. The date of maximum abundance increased with latitude such that Daphnia peaked during nearly all months of the year depending on latitude, but usually within the same, narrow temperature range (15‐208C). The period of minimum Daphnia abundance decreased linearly with latitude such that abundance generally was minimal when water temperatures exceeded 208C. Cladocerans are important to aquatic ecosystem function because they include the primary herbivores in lakes (Daphnia) and are important diet items for fishes (Dodson and Hanazato 1995). Attributes of cladoceran communities, including the mean body size and population dynamics of species, are often considered to be most influenced by biotic factors (e.g., competition, predation), largely on the basis of investigations of individual lakes in temperate regions. A comparison of cladoceran communities across lakes may provide a valuable new understanding of the factors influencing the mean body size and population dynamics of cladoceran communities by highlighting the role of abiotic factors operating at larger scales (e.g., climate). Specifically, a comparison of patterns in cladoceran communities in lakes across latitudes provides an opportunity to consider the relationship of temperature to the mean body size and population dynamics of cladocerans. Temperature has been shown to be critical to the survival, growth, and reproduction of cladocerans (Allan and Goulden 1980; Goss and Bunting 1983; Moore et al. 1996), yet it is seldom recognized as an important influence on the body size or population dynamics of cladoceran communities. The presence of broad-scale patterns in cladoceran communities across lakes has rarely been recognized, or even explored, at regional scales (exceptions: Kratz et al. 1987; Wiseman 1996). Although within temperate lakes, a recurring seasonal pattern is widely recognized whereby Daphnia populations increase in spring, decline in midsummer, and increase again in the fall (Hutchinson 1967; Culver 1980; Threlkeld 1985). Here we evaluate whether patterns in cladoceran body size and population dynamics are present across latitudes. We begin with an analysis of published data to explore patterns in the mean body size of cladocerans

Planktonic production and respiration in oligotrophic Shield lakes

Abstract: A precise oxygen method was used to measure primary production, community respiration and to determine the importance of exogenous organic carbon as an energy source to planktonic communities in the epilimnion of 12 oligotrophic to mesotrophic Shield lakes. Median photosynthetic parameters observed with the oxygen method were up to twice as high as those measured with 14 C in other oligotrophic Shield lakes. Gross photosynthesis was almost always larger than community respiration, with a median P : R ratio of 1.7. We observed strong relationships between respiration and gross photosynthesis, but could not detect any significant trend between respiration or the P : R ratio and the concentration of dissolved organic carbon (DOC). DOC appeared to depress both photosynthesis and respiration. These results argue against the importance of exogenous organic carbon supply as a significant energy source to freshwater planktonic communities. Previously low P : R ratios reported for oligotrophic fresh waters may be due to the uncertain meaning of 14 C production data.

Evolutionary and ecological perspectives on carbon acquisition in phytoplankton

Abstract: Statistically significant differences exist among the major algal and cyanobacterial taxa in the catalytic efficiency of Ribulose-1,5 bisphosphate carboxylase/oxygenase (RubisCO)—the rate limiting carboxylase in C-fixation. With the notable exception of the red algae, a comparison of the kinetic properties of RubisCO among phytoplankton of different geological ages suggests that the enzyme has evolved towards higher CO2 affinity in response to decreasing atmospheric CO2/O2 ratios over geological time. Taxonomic differences in the kinetic properties of RubisCO influence the mechanisms of C acquisition among phytoplankton groups. A significant negative correlation exists between the CO2 specificity factor of RubisCO and the capacity of cells to concentrate inorganic C through active uptake mechanisms. Differential resource requirements associated with C acquisition could affect the relative fitness of phytoplankton groups and thus their ecological interactions. I argue that inorganic C may play a more important role in phytoplankton productivity and community ecology than has been previously recognized and highlight some outstanding questions for future research.

Domoic acid production near California coastal upwelling zones, June 1998

Abstract: Sea lion mortalities in central California during May and June 1998 were traced to their ingestion of sardines and anchovies that had accumulated the neurotoxin domoic acid. The detection of toxin in urine, feces, and stomach contents of several sea lions represents the first proven occurrence of domoic acid transfer through the food chain to a marine mammal. The pennate diatoms, Pseudo-nitzschia multiseriesand P. australis, were the dominant, toxinproducing phytoplankton constituting algal blooms near Monterey Bay, Half Moon Bay, and Oceano Dunes, areas where sea lions with neurological symptoms stranded. Toxic Pseudo-nitzschia were also found near Morro Bay, Point Conception, Point Arguello, and Santa Barbara, demonstrating that these species were widespread along the central California coast in June 1998. Measurements of domoic acid during three cruises in early June showed the )†