Linkage between microzooplankton grazing and phytoplankton growth in a Gulf of Mexico estuary

Microzooplankton dilution grazing experiments were conducted with water collected from Pensacola Bay, Florida (USA) on 12 dates at 2 sites. Statistically significant grazing rates were observed in 22 of 24 experiments. Grazing rates in Upper Bay and Lower Bay were similar averaging 0.54 and 0.51 d⁻¹, respectively. Phytoplankton growth rates were also similar at the two sites, averaging 1.02 and 1.00 d⁻¹ at Upper Bay and Lower Bay, respectively. Phytoplankton growth rates usually exceeded grazing rates by about a factor of two, though microzooplankton grazing represented a significant mortality for phytoplankton. The literature suggests a linkage between phytoplankton growth and microzooplankton grazing that spans a wide variety of aquatic environments. While individual growth and grazing rates were variable, growth frequently exceeded grazing by about two-fold. This implies that the role of microzooplankton is similar across a wide variety of aquatic systems.     

Phytoplankton Growth Balanced by Clam and Zooplankton Grazing and Net Transport into the Low-Salinity Zone of the San Francisco Estuary

We estimated the influence of planktonic and benthic grazing on phytoplankton in the strongly tidal, river-dominated northern San Francisco Estuary using data from an intensive study of the low salinity foodweb in 2006–2008 supplemented with long-term monitoring data. A drop in chlorophyll concentration in 1987 had previously been linked to grazing by the introduced clam Potamocorbula amurensis, but numerous changes in the estuary may be linked to the continued low chlorophyll. We asked whether phytoplankton continued to be suppressed by grazing and what proportion of the grazing was by benthic bivalves. A mass balance of phytoplankton biomass included estimates of primary production and grazing by microzooplankton, mesozooplankton, and clams. Grazing persistently exceeded net phytoplankton growth especially for larger cells, and grazing by microzooplankton often exceeded that by clams. A subsidy of phytoplankton from other regions roughly balanced the excess of grazing over growth. Thus, the influence of bivalve grazing on phytoplankton biomass can be understood only in the context of limits on phytoplankton growth, total grazing, and transport.     

Microzooplankton grazing and nitrogen excretion across a surface estuarine-coastal interface

The role of the microzooplankton community in regulating phytoplankton biomass was examined across a gradient from a river-dominated estuary to an oceanic-influenced coastal zone. Three stations located along a salinity gradient from the central region of Mobile Bay to 10 km off the coast were sampled from May 1994 to August 1995. Microzooplankton herbivory rates on phytoplankton and microzooplankton excretion of nitrogen derived from phytoplankton were estimated using the dilution technique. Microzooplankton grazing rates (range of station means=0.57–1.10 d⁻¹) and phytoplankton growth rates (0.70–1.62 d⁻¹) both increased across the salinity gradient from the bay station to the offshore station. However, the percent of primary production grazed per day was highest at the bay station (mean=83%) and decreased to a low at the offshore station (mean=64%). Excretion of phytoplankton-derived nitrogen by the microzooplankton was greatest at the bay and bay mouth stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stations, respectively. These results support the idea that herbivorous microzooplankton are important in mediating nitrogen flow to both lower and higher trophic levels. *** DIRECT SUPPORT *** A01BY085 00012     

海洋浮游生态系统中小型浮游动物的生态功能

小型浮游动物在海洋生态系统中的作用,主要指有多少能量通过小型浮游动物传递到桡足类,从而比较沿"浮游植物→中型浮游动物"和"浮游植物+细菌→小型浮游动物→中型浮游动物"两条食物链到达中型浮游动物的能流大小.为达到这个目的,需要研究各个能流路径的传递效率,即能量在各个营养级(初级生产--小型浮游动物,细菌生产--小型浮游动物,小型浮游动物的生长率,小型浮游动物--中型浮游动物)的传递效率.综述了国内外对上述营养级传递效率的研究现状,以期为我国的同类研究提供参考.浮游植物初级生产力被小型浮游动物摄食的比例平均为每天60%～75%,大大高于桡足类对浮游植物初级生产力的摄食压力每天10%.海洋浮游细茵的二次生产力相当于初级生产力的30%.其中80%～180%被小型浮游动物摄食.小型浮游动物的毛生长率为30%～40%,生产力是初级生产力的21%～34%.在西班牙西北部沿海,桡足类每天摄食2%～51%小型浮游动物生产力.因此,桡足类通过微食物网摄食的能量是初级生产力的0.4%～17%,与桡足类摄食初级生产的10%处于同一量级.不考虑碎屑提供的能流,小型浮游动物对桡足类饵料的贡献为20%以上,甚至可高达50%.     

Changes in plankton community structure and function in response to variable freshwater flow in two tributaries of the Chesapeake Bay

The biomass of phytoplankton, microzooplankton, copepods, and gelatinous zooplankton were measured in two tributaries of the Chesapeake Bay during the springs of consecutive dry (below average freshwater flow), wet (above average freshwater flow), and average freshwater flow years. The potential for copepod control of microzooplankton biomass in the dry and wet years was evaluated by comparing the estimated grazing rates of microzooplankton by the dominant copepod species (Acartia spp. andEurytemora affinis) to microzooplankton growth rates and by calculating the percent of daily microzooplanton standing stock removed through copepod grazing. There were significant increases in phytoplankton and copepod biomass, but not for microzooplankton biomass in the wet year as compared to the dry year. The ctenophoreMnemiopsis leidyi was present during the dry year but was absent during the sampling period of the wet and average freshwater flow years. Grazing pressure on microzooplankton was greatest in the wet year, withAcartia spp. andE. affinis ingesting 0.21–2.64 μg of microzooplankton C copepod⁻¹ d⁻¹ and removing up to 60% of the microzooplankton standing stock per day. In the dry year, these copepod species ingested 0.10–0.73 μg of microzooplankton C copepod⁻¹ d⁻¹ with a maximum daily removal of approximately 3% of the microzooplankton standing stock. Potential copepod grazing pressure was significantly less than microzooplankton growth in the dry year, but was equivalent to microzooplankton growth in the wet year, implying strong top-down control of the microzooplankton community in the wet year. These results suggest that increased grazing control of microzooplankton populations by more copepods in the wet year released top-down control of phytoplankton. Reduced microzooplankton grazing, in conjunction with increased nutrient availability, resulted in large increases in phytoplankton biomass in the wet year. Increased freshwater flow has the potential to influence trophic cascades and the partitioning of plankton production in estuarine systems.     

Factors Controlling Phytoplankton Biomass in a Subtropical Coastal Lagoon: Relative Scales of Influence

Patterns in phytoplankton biomass are essential to understanding estuarine ecosystem structure and function and are the net result of various gain and loss processes. In this study, patterns in phytoplankton biomass were explored in relation to a suite of potentially regulating factors in a well-flushed, subtropical lagoon, the Matanzas River Estuary (MRE) in northeast Florida. We examined temporal variability in water temperature, light availability, nutrient concentrations, phytoplankton productivity, and phytoplankton standing stock over 8 years (2003–2010) and explored relationships among variables through correlation analysis. Laboratory experiments in the spring and summer of 2009 quantified phytoplankton growth rates, nutrient limitation potential, and zooplankton grazing rates. The potential influence of oyster grazing was also examined by scaling up population metrics and filtration rate estimates. Results indicated that phytoplankton biomass in the study area was relatively low mainly due to a combination of low temperature and light availability in the winter and consistent tidal water exchange and bivalve grazing throughout the year. Relatively low levels of phytoplankton standing stock and small inter-annual variability within the MRE reflect a balance between gain and loss processes which provide a degree of resilience of the system to natural and anthropogenic influences.     

The Role of Protistan Microzooplankton in the Upper San Francisco Estuary Planktonic Food Web: Source or Sink?

Decline of native pelagic species in estuarine systems is an increasing problem, especially for native fishes in the San Francisco Estuary and Delta (SFE-D). Addressing these losses depends on understanding trophodynamics in the food web that supports threatened species. We quantified the role of microzooplankton (heterotrophic–mixotrophic protists <200 μm) in the food web of the upper SFE-D. We sampled protist plankton abundance and composition at two sites (Suisun Bay and Grizzly Bay) approximately monthly from February 2004 to August 2005 and conducted dilution experiments during spring and summer of both years in Suisun Bay. Heterotrophs dominated the protist community in Suisun Bay and Grizzly Bay, particularly in the <20 μm size range, and peaks in protistan microzooplankton biomass were associated with high phytoplankton biomass. In both years, microzooplankton grazing rates were high (0.5–0.7 day⁻¹) during the spring and lower (~0.2 day⁻¹) during summer. Phytoplankton growth rates peaked in April 2004 (~0.7 day⁻¹) but were much lower (<0.1 day⁻¹) in spring 2005, despite relatively high abundance. Thus, microzooplankton grazing consumed as much as 73% of phytoplankton standing stock during spring and ~15% of standing stock during summer of both years. Combined with earlier results, we conclude that microzooplankton can be important mediators of carbon and energy flow in the upper SFE-D and may be a “source” to the metazoan food web.     

Effects of zooplankton grazing on phytoplankton size-structure and biomass in the lower Hudson River estuary

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various¹⁴C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m^?2 h^?1/mg C produced m^?2 h^?1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.     

Trophic Links in the Plankton in the Low Salinity Zone of a Large Temperate Estuary: Top-down Effects of Introduced Copepods

We investigated trophic relationships involving microzooplankton in the low salinity zone of the San Francisco Estuary (SFE) as part of a larger effort aimed at understanding the dynamics of the food web supporting the endangered delta smelt, Hypomesus transpacificus. We performed 14 cascade experiments in which we manipulated the biomass of a copepod (Limnoithona tetraspina, Pseudodiaptomus forbesi, or Acartiella sinensis) and quantified responses of lower trophic levels including bacterioplankton, phytoplankton, and microzooplankton. Microzooplankton comprised a major food source for copepods; 9 out of 14 experiments showed removal of at least one group of microzooplankton by copepods. In contrast, the impact of copepods on phytoplankton was indirect; increased copepod biomass led to greater growth of phytoplankton in 3 of 14 experiments. Estimated clearance rates on microzooplankton were 4 mL day^?1 for L. tetraspina and 2–6 mL day^?1 for P. forbesi, whereas A. sinensis consumed mainly copepod nauplii. Complex trophic interactions, including omnivory, among copepods, microzooplankton, and different components of the phytoplankton likely obscured clear trends. The food web of the SFE is probably less efficient than previously thought, providing poor support to higher trophic levels; this inefficient food web is almost certainly implicated in the continuing low abundance of fishes, including the delta smelt that use the low salinity zone of the San Francisco Estuary.     

The Role of Zooplankton Grazing and Nutrient Loading in the Occurrence of Harmful Cyanobacterial Blooms in Florida Bay,USA

Florida Bay is Florida’s (USA) largest estuary and has experienced harmful picocyanobacteria blooms for nearly two decades. While nutrient loading is the most commonly cited cause of algal blooms in Florida Bay, the role of zooplankton grazing pressure in bloom occurrence has not been considered. For this study, the spatial and temporal dynamics of cyanobacteria blooms, the microbial food web, microzooplankton and mesozooplankton grazing rates of picoplankton, and the effects of nutrients on plankton groups in Florida Bay were quantified. During the study, cyanobacteria blooms (>3 × 10⁵ cells mL⁻¹) persisted in the eastern and central regions of Florida Bay for more than a year. Locations with elevated abundance of cyanobacteria hosted microzooplankton grazing rates on cyanobacteria that were significantly lower (p < 0.001) and less frequently detectable compared to sites without blooms. Consistent with this observation, cyanobacteria abundances were significantly correlated with ciliates and heterotrophic nanoflagellates at low cyanobacteria densities (p < 0.001) but were not correlated during bloom events. The experimental enrichment of mesozooplankton abundance during blooms yielded a significant decrease in the net growth rate of picoplankton but had the opposite effect when blooms were absent, suggesting that the cascading effect of mesozooplankton grazing on the microbial food web was also altered during blooms. While inorganic nutrient enrichment significantly increased the net growth rates of eukaryotic phytoplankton and heterotrophic bacteria, such nutrient loading had no effect on the net growth rates of cyanobacteria. Hence, this study demonstrates that low rates of zooplankton grazing and low rates of inorganic nutrient loading contribute to the persistence of cyanobacteria blooms in Florida Bay.     

Zooplankton grazing in a Potomac River cyanobacteria bloom

During summer, bloom-forming cyanobacteria, including Anacystis, Aphanizomenon, and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 10⁸ cells l^?1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing experiments were conducted in July and August 1987. Using size-fractionated river phytoplankton assemblages, zooplankton grazing rates were determined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on ¹⁴C-labeled river assemblages, ¹⁴C-labeled river assemblages enriched with unlabeled cyanobacteria, and unlabeled river assemblages enriched with ¹⁴C-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mira, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trichocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither rotifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl· [individual · h]^?1 while copepod rates ranged from undetectable to 135 μl · [copepod · h]^?1. Grazing rates on labeled river phytoplankton assemblages were 4–100 times higher than noted for zooplankton feeding on cyanobacteria. The addition of the colonial alga to labeled river phytoplankton assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no added cyanobacteria. Total zooplankton demand for cyanobacteria and river phytoplankton assemblages was estimated for the study period July–August 1987. Rotifer plus C. vernalis herbivory would have removed 1–5% and 49%, respectively, of the standing stock of the two autotroph pools each day. Literature-derived clearance rates for Bosmina indicate, however, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungrazed and may be transported to the lower estuary for salinity-induced aggregation and sedimentation.     

Selective feeding and grazing rates of oyster (Crassostrea virginica) larvae on natural phytoplankton assemblages

Results of laboratory feeding experiments suggest that oyster (Crassostrea virginica) larvae select small phytoplanktonic organisms (<10 μm) over larger-celled forms from natural estuarine assemblages, but that little selection occurs within the small phytoplankton fraction. Laboratory-reared larvae grazed each of five small-celled phytoplankton groups enumerated (coccoids, centrate diatoms, pennate diatoms, dinoflagellates, and flagellates) at rates proportional to phytoplankton group densities in controls (no larvae added). Larval grazing rates (number of cells removed per larva per hour) were inversely proportional to both larval density and experimental duration.     

Effect of fish and mesozooplankton manipulation on the phytoplankton community in the Patos Lagoon estuary,Southern Brazil

An enclosure experiment in the Patos Lagoon estuary, southern Brazil (32°S, 52°W) investigated the response of phytoplankton to the enhancement of mesozooplankton and fish abundance. Addition of nutrients (NO₃ ^? and PO₄ ^?) stimulated the growth of 3–20 μm diameter phytoplankton, especially the diatomCylindrotheca closterium, which, in turn, was heavily grazed by the dominant mesozooplankter, the copepodAcartia tonsa. Acartia did not consume small (2–3 μm) autotrophic flagellates and the cyanobacteriaAnabaena sp., despite their high cell number. Largest grazing ofC. closterium by the copepod occurred only after a decrease of ciliate abundance. The addition of mesozooplankton did not change the levels of primary production, but it significantly increased the phytoplankton assimilation number. Highest chlorophylla concentrations were measured in enclosures stocked with juveniles of the fishXenomelaniris brasiliensis. The large phytoplankton biomass and low mesozooplankton abundance found in this treatment indicates an, effective predator-mediated action on the phytoplankton community.     

Barium uptake and adsorption in diatoms

Using trace metal-defined culture conditions, we measured the cellular barium concentration in cultures of the diatom Thalassiosira weissflogii. In cultures with low Fe concentration, at a typical surface seawater Ba concentration of 35 to 40 nmol/L, the cellular Ba was 0.5 μmol/mol P (∼0.04 μg/g dw), much below all previously published values for phytoplankton. When the Fe concentration in the growth medium was increased, the measured “cellular” Ba increased proportionally to the particulate Fe. Under conditions of ferric hydroxide precipitation, similar particulate Ba concentrations were measured in the absence and the presence of cells for a given concentration of particulate Fe. Thus, the bulk of the particulate Ba in such experiments was not truly intracellular but adsorbed on precipitated Fe hydroxides associated with the cell surface. A similar mechanism likely explains the high cellular Ba concentrations reported by previous investigators who utilized relatively high ratios of Fe to EDTA concentrations in the growth medium, resulting in Fe precipitation. On the basis of our results, it seems highly unlikely that intracellular uptake by living phytoplankton explains the vertical flux of Ba in oceanic waters. But the adsorption of Ba on Fe hydroxides that is seen in laboratory experiments must also occur in the oceans and contribute to Ba transport in the upper part of the water column. To be significant, however, such a mechanism would require that the particles that transport Ba out of the surface have a markedly lower Ba/Fe ratio than is measured in the average suspended material.     

Increasing Severity of Phytoplankton Nutrient Limitation Following Reductions in Point Source Inputs to the Tidal Freshwater Segment of the James River Estuary

We conducted monthly bioassay experiments to characterize light and nutrient use efficiency of phytoplankton communities from the chlorophyll-a maximum located in the tidal freshwater region of the James River Estuary. Bioassay results were interpreted in the context of seasonal and inter-annual variation in nutrient delivery and biomass yield using recent and long-term data. Bioassay experiments suggest that nutrient limitation of phytoplankton production has increased over the past 20 years coinciding with reductions in point source inputs and estuarine dissolved nutrient concentrations. Despite increasing nutrient stress, chlorophyll concentrations have not declined due to more efficient nutrient usage. Greater CHLa yield (per unit of N and P) may be due to feedback mechanisms by which the presence of toxin-producing cyanobacteria inhibits grazing by benthic and pelagic filter-feeders. Seasonal patterns in nutrient limitation indicate that phytoplankton in the James respond to variations in inflow concentrations of dissolved nutrients. This association gives rise to an atypical pattern whereby the severity of nutrient limitation diminishes with low discharge in late summer due to minimal dilution of local point sources inputs by riverine discharge. We suggest that this may be a common feature of estuaries located in proximity to urbanized areas.     

In Situ Response of Phytoplankton to Nutrient Additions in a Tropical Coastal Lagoon, (La Mancha,Veracruz, Mexico)

An experimental in situ microcosm study was conducted in the tropical lagoon La Mancha (Gulf of Mexico) to determine whether or not nutrient limitation occurs and to examine the direct effect of an inorganic nutrient pulse on the phytoplankton community structure. The phytoplankton community response to the addition of four treatments with different combinations of nitrogen (N), phosphorus (P), and silica (Si) (+N-NH₄ ⁺, +P-PO₄ ^?, +Si-SO₃, and N:P16) showed that phytoplankton was N-limited as indicated by an increase in phytoplankton biomass (i.e., chlorophyll a) (range, 8–34 mg m^?3) during the dry season in two consecutive years (2006 and 2007). Picophytoplankton abundance significantly increased in the +N treatment (145.46 10³ cells L^?1), while microphytoplankton reached a maximum abundance (68.38 10³ cells L^?1) in the N:P16 treatment. Phytoplankton composition changed from a community initially dominated by dinoflagellates (e.g., Prorocentrum spp.) to another dominated by diatoms (Thalassiosira and Nitzschia longissima) in the N:P16 treatment. The +N treatment significantly increased Synechococcus sp. growth rates (1.3 divisions per day) (picocyanobacteria). Biomarker pigments measured in the experimental microcosms confirmed observed changes in phytoplankton groups. Our results reveal that La Mancha lagoon is a N-limited coastal system during the dry season and provides evidence of the temporal species successional patterns and mechanisms regulating the phytoplankton community response to nutrient enrichment pulses in this already eutrophic coastal lagoon.     

Primary productivity and phytoplankton size fraction dominance in a temperate North Atlantic estuary

The composition, productivity, and standing crop of net (>20 μm) and nano-(<20 μm) phytoplankton of Peconic Bay, Long Island, New York was examined from June 1978 through May 1979. Nanoplankton, primarily small solitary flagellates, chlorophytes, and diatoms, dominated from May through September accounting for 88.5% of the productivity and 88.1% of the standing crop (measured as chlorophyll a). An apparent net plankton bloom began in December and continued through March. The dominant organism through most of the winter bloom was the chain-forming diatom Skeletonema costatum (Grev.) Cl. Net plankton at this time represented 66.4% of the standing crop. For both size fractions, productivity/chlorophyll a (g C per g chl a per d, integrated through the euphotic zone) was a function of light energy over the year with the exception of a few sampling dates during the post-winter bloom period. Assimilation numbers (g C per g chl a per h at saturating light intensities) were a function of temperature between 0 and 20°C. Nitrogen deficiency did not appear to be a factor in regulating phytoplankton growth rate through the euphotic zone, as ratios of ¹⁴C assimilation for dark bottles enriched with NH₃ and with no enrichment exhibited no relationship to environmental dissolved inorganic nitrogen concentrations. Zooplankton grazing pressure appeared to have been an important factor in regulating the upper limit of phytoplankton biomass and in influencing size fraction dominance. Dominance of one phytoplankton size fraction over the other on any given date was not based on physiological differences between the two groups since both fractions were composed of the same species. Apparent net phytoplankton blooms (in terms of productivity and chlorophyll a) were artifacts of increased chain lengths of nanoplankton diatoms such as Skeletonema costatum, and to a lesser extent, Thalassiosira nordenskioldii Cl. and Detonula confervacea (Cl.) Gran, rather than to the dominance of large, solitary cells.     

Short-term biogeochemical influence of a diatom bloom on the nutrient and trace metal concentrations in South San Francisco Bay microcosm experiments

Two laboratory microcosm experiments were conducted to mimic an annual spring diatom bloom in South San Francisco Bay by isolating the phytoplankton community from the benthic grazing pressure to induce a phytoplankton bloom. The purpose of these experiments was to isolate the impact of a spring diatom bloom on the nutrient and trace metal geochemical cycling. Microcosms were created in 2.5 L incubation bottles and subjected to one of 4 treatments (control, copper [Cu] addition, manganese [Mn] addition, and both Cu and Mn addition) to investigate the toxicity of Cu on the resident plankton and the potential antagonistic effects of Mn on reducing Cu toxicity. Dissolved macronutrient (nitrate + nitrite, phosphate, and silicate), and dissolved and particulate trace metal (Cu, Ni, Mn) concentrations were monitored in the grow-out incubations on a daily basis. Chlorophylla concentrations were also monitored over the course of the experiment and used to calculate diatom-specific growth rates. In the experiments containing ambient South San Francisco Bay surface waters, average specific growth rates were on the order of 1.1 d^?1. The induced diatom blooms resulted in significant removal of macronutrients from the microcosms over the course of the experiments. Our research supports previous suggestions that dissolved Ni and Cu concentrations in South San Francisco Bay have a very low biological availability as a result of organic chelation. Ni(EDTA)^2? has been found to be the dominant dissolved Ni species by other researchers and Cu speciation analyses from this study and others indicate that > 99% of the dissolved Cu in South San Francisco Bay is strongly chelated as CuL₁. The free cupric ion concentration was on the order of 10^?12 M. Marked removal of dissolved Mn was observed in the control treatments, well exceeding expected dissolved Mn removal by diatom uptake. Additions of 375 nM Cu resulted in the complete titration of the chelating ligand (L₁) concentrations. The elevated [Cu²⁺] (≈10^?8MM) appeared to have a toxic effect on the diatom community observed in the significant decreases in their specific growth rates (μ=0.4 d^?1). The suppression of dissolved Mn removal from solution was also observed in treatments spiked with high levels of dissolved Cu, providing support that Mn precipitation was due to biologically mediated oxidation not phytoplankton assimilation. The observed geochemical behavior in the concurrent Cu and Mn addition treatments provide evidence in support of Mn alleviation of Cu toxicity. The biological role in the ambient short-term biogeochemical cycling of Cu and Ni in South San Francisco Bay appears to be minimal due to the inert character of the organic ligand-metal complexes. A significant portion of the annual macronutrient and Mn cycling occurs as a result of spring diatom blooms in South San Francisco Bay.     

Climatic Trends and Temporal Patterns of Phytoplankton Composition, Abundance, and Succession in the Indian River Lagoon, Florida, USA

This paper describes the results of 10 years of water quality monitoring in the Indian River Lagoon Florida, with special emphasis on the relationships between trends in climatic conditions and the distribution, composition, and abundance of the phytoplankton community. The Indian River Lagoon, which spans 220 km of Florida’s east coast, is a region of particular concern because of the rapid rate of human development throughout the region and the hydrologically restricted character of the lagoon, which heightens the potential for algal bloom. Water sampling was carried out on a monthly to twice-monthly basis at six sites located in the northern and central lagoon. The 10-year study included both extended periods of below and above average rainfall. A number of ecologically distinct regions exist within the lagoon, which differ considerably in water exchange properties and watershed inputs. The northern lagoon is characterized by longer water residence times, lower phosphorus concentrations, higher nitrogen concentrations, and more stable salinity conditions than the central lagoon. Mean phytoplankton biovolumes were substantially higher at the sites in the northern lagoon than at the sites in the central lagoon, and algal blooms were more common and intense in the former region. Inter-annual patterns of phytoplankton biovolume were also different in the northern and central lagoon. In the northern lagoon, phytoplankton biovolumes were lowest during the drought period, from the autumn of 1998 to the spring of 2001. By contrast, algal bloom events in the central lagoon were not only less frequent but were not tied to periods of high rainfall. The most widespread and common bloom formers were the potentially toxic dinoflagellate Pyrodinium bahamense var. bahamense and two centric diatoms, Dactyliosolen fragilissimus and Cerataulina pelagica. Many of the biovolume peaks observed over the study period were attributable to these three species. The results of time series modeling of phytoplankton dynamics further highlighted the disparities between the two regions of the lagoon in terms of the suite of parameters that best predict the observed trends in the biomass of phytoplankton. Overall, the outcome of this initial modeling effort in the Indian River Lagoon suggests that time series approaches can help define the factors that influence phytoplankton dynamics.     

Microzooplankton Grazing in Green Water—Results from Two Contrasting Estuaries

We measured seasonal variations in microzooplankton grazing in Long Island Sound (LIS) and San Francisco Bay (SFB). There was consistent evidence of nutrient limitation in LIS, but not SFB. We found higher chlorophyll a concentrations in LIS compared with SFB. In spite of differences in phytoplankton, there were no differences in microzooplankton abundance (summer: LIS, 12.4 ± 1.8 × 10³ indiv. L⁻¹; SFB, 14.1 ± 3.0 × 10³ indiv. L⁻¹), biomass (summer: LIS, 30.4 ± 5.0 μg C L⁻¹; SFB, 26.3 ± 5.9 μg C L⁻¹), or grazing rates (summer: LIS, 0.66 ± 0.19 day⁻¹; SFB, 0.65 ± 0.18 day⁻¹) between the two estuaries. In common with many other investigators, we found many instances of saturated as well as insignificant grazing. We suggest that saturation in some cases may result from high particle loads in turbid estuarine systems and that insignificant grazing may result from extreme saturation of the grazing response due to the need to process non-food particles.