Grazing Rates Research Articles

Different responses of plankton community to mesoscale eddies in the western equatorial Pacific Ocean

The western equatorial Pacific Ocean exhibits a complex pattern of currents and mesoscale eddies. However, the impacts of dynamic hydrological systems on plankton community remain uncertain due to the low resolution of field observation. In this study, we conducted a comparative study involving regional high-resolution field observations, satellite observations, and the numerical model. Our aim was to clarify the spatial distribution characteristics and underlying factors influencing phytoplankton communities in the western equatorial Pacific Ocean under different currents and mesoscale eddies. The combination of field observations and numerical model suggested that the Mindanao Eddy (cyclonic eddy) exhibited a significantly higher total chlorophyll a concentration than the North Equatorial Current, and the contributions of diatom increased as well. In the subtropical gyre, although the intensity of a transient cyclonic eddy was weak, it still lifted the subsurface phytoplankton up and increased their chlorophyll a concentration. In the Halmahera Eddy (anti-cyclonic eddy), however, Prochlorococcus and haptophytes were the dominant groups, with lower total chlorophyll a compared to the surrounding North Equatorial Counter Current. Generally, the distributions of these phytoplankton communities were primarily influenced by variations in temperature and nutrient concentrations. Interestingly, this study further revealed that both cyclonic and anticyclonic eddies enhanced the grazing rate of microzooplankton through the increasing of prey concentration and temperature, respectively. These factors driven by different eddies affect phytoplankton communities directly or indirectly through bottom-up and top-down controls, and demonstrate the impact of different mesoscale eddies on phytoplankton community in the western equatorial Pacific Ocean, which is a region of great significance to both the regional and global ecosystems.

Microzooplankton Grazing Exerts a Strong Top‐Down Control on Unicellular Cyanobacterial Diazotrophs

AbstractUnicellular cyanobacterial diazotrophs (UCDs) are important nitrogen (N) fixers in marine ecosystems. However, the top‐down control on UCDs is poorly understood, especially for microzooplankton grazing that is often considered to be the dominant loss factor for primary production in oligotrophic oceans. In this study, we investigated the growth and microzooplankton grazing rates of the major UCD taxa (UCYN‐A1, UCYN‐A2/3/4, UCYN‐B, and UCYN‐C) in the South China Sea and the Luzon Strait using a combination of dilution experiments and quantitative PCR. All UCD taxa were actively grazed by microzooplankton, with higher grazing rates occurring in deeper layers of the photic zone. Microzooplankton grazing rates of UCDs were significantly higher than those of phytoplankton as whole, indicating that UCDs might be preferentially grazed by microzooplankton assemblages. The growth and microzooplankton grazing rates of UCDs were almost balanced, suggesting that a substantial amount of the UCD‐derived N is transferred through the food web via microzooplankton in these regions. Our study highlights the importance of microzooplankton grazing in the fate of UCD‐derived N and indicates that UCDs contribute significantly to the plankton food web in oligotrophic oceans where new N is mainly originated from dinitrogen fixation.

Shift in algal blooms from micro‐ to macroalgae around China with increasing eutrophication and climate change

AbstractBlooms of microalgal red tides and macroalgae (e.g., green and golden tides caused by Ulva and Sargassum) have caused widespread problems around China in recent years, but there is uncertainty around what triggers these blooms and how they interact. Here, we use 30 years of monitoring data to help answer these questions, focusing on the four main species of microalgae Prorocentrum donghaiense, Karenia mikimotoi, Noctiluca scintillans, and Skeletonema costatum) associated with red tides in the region. The frequency of red tides increased from 1991 to 2003 and then decreased until 2020, with S. costatum red tides exhibiting the highest rate of decrease. Green tides started to occur around China in 1999 and the frequency of green tides has since been on the increase. Golden tides were first reported to occur around China in 2012. The frequency of macroalgal blooms has a negative linear relationship with the frequency and coverage of red tides around China, and a positive correlation with total nitrogen and phosphorus loads as well as with atmospheric CO2 and sea surface temperature (SST). Increased outbreaks of macroalgal blooms are very likely due to worsening levels of eutrophication, combined with rising CO2 and SST, which contribute to the reduced frequency of red tides. The increasing grazing rate of microzooplankton also results in the decline in areas affected by red tides. This study shows a clear shift of algal blooms from microalgae to macroalgae around China over the past 30 years driven by the combination of eutrophication, climate change, and grazing stress, indicating a fundamental change in coastal systems in the region.

Coexisting picoplankton experience different relative grazing pressures across an ocean productivity gradient.

Picophytoplankton populations [Prochlorococcus, Synechococcus (SYN), and picoeukaryotes] are dominant primary producers in the open ocean and projected to become more important with climate change. Their fates can vary, however, with microbial food web complexities. In the California Current Ecosystem, picophytoplankton biomass and abundance peak in waters of intermediate productivity and decrease at higher production. Using experimental data from eight cruises crossing the pronounced CCE trophic gradient, we tested the hypothesis that these declines are driven by intensified grazing on heterotrophic bacteria (HBAC) passed to similarly sized picophytoplankton via shared predators. Results confirm previously observed distributions as well as significant increases in bacterial abundance, cell growth, and grazing mortality with primary production. Mortalities of picophytoplankton, however, diverge from the bacterial mortality trend such that relative grazing rates on SYN compared to HBAC decline by 12-fold between low and high productivity waters. The large shifts in mortality rate ratios for coexisting populations are not explained by size variability but rather suggest high selectivity of grazer assemblages or tightly coupled tradeoffs in microbial growth advantages and grazing vulnerabilities. These findings challenge the long-held view that protistan grazing mainly determines overall biomass of microbial communities while viruses uniquely regulate diversity by "killing the winners".

Grazing pressure of sea urchin Mesocentrotus nudus estimated from seawater temperature and wave height, and its effects on the early growth of annual kelp Saccharina japonica in northeastern Japan

Grazing by the sea urchin Mesocentrotus nudus plays a vital role in the development of marine forests of annual kelp, Saccharina japonica, along the Pacific coast of northeastern Japan. The grazing rate of sea urchins, which are rigorously temperature dependent, affects the chance of germination/survival of the kelp during winter/spring when seawater temperatures in this region are highly variable among years. It has previously been found that if the mean seawater temperature between February and March remained below 5.5 °C, kelp forests in this region would establish later in the year. However, the mechanism leading to this success or failure of kelp forests to develop has yet to be investigated in detail.Laboratory experiments confirmed that the grazing rate and movement of the sea urchin decreased as seawater temperature declined from 12 °C to 3 °C. Field observations, using interval cameras from autumn to spring, revealed that the density of sea urchins on artificial reefs decreased at lower seawater temperatures and/or higher significant wave heights (SWH). Using these relationships, the grazing pressure by sea urchins (= grazing rate × density) in the site was estimated based on seawater temperature and SWH. Comparing the grazing pressures among past years with different states of kelp forests, conditions needed to establish kelp forests were revealed. Extremely low grazing pressure (< 0.5 g m−2 day−1) lasted for a total of 31 days in 2015 when kelp grew densely, while this condition occurred on 11 and 14 days respectively in 2019 and 2022 when kelp was sparse. Kelp did not grow in 2016 when the grazing pressure continuously remained above 0.5 g m−2 day−1. At this study site, such combinations of seawater temperature and SWH as 4 °C and 1.0 m, 6 °C and 2.3 m, and 8 °C and 3.7 m would equate to a grazing pressure of 0.5 g m−2 day−1. To develop a dense kelp forest, grazing pressure was required to be lower than 0.5 g m−2 day−1 for about a month as in 2015. Though these figures should be adjusted, a model and scheme of this study will be applied to the other areas where kelp forest restoration/management plans are required.

Is a lower-toxicity strain of Microcystis aeruginosa really less toxic?

Some well-known hazards of blooming cyanobacteria are caused by toxic metabolites such as microcystins (MCs), though many other bioactive chemicals of unknown toxicity are present in their exudates. It is also unclear whether toxicity of cyanobacterial cells depends on growth phases in the life cycle. In this study, we compared toxicity to Daphnia magna of Microcystis aeruginosa - a common cyanobacterial species - exudates (MaE) from two MC-producing strains over both exponential growth and stationary phases in acute and chronic experiments. Specifically, we assessed mitochondrial dysfunction, oxidative stress and lipid peroxidation, and filtering activity and heartbeat rate of Daphnia exposed to MaE. All MaE treatments induced common characteristics of Microcystis toxicity including disorder in the mitochondrial membrane and aberrant heart rate. MaE from cells at stationary growth phase were more toxic than those at exponential phase. Surprisingly, the MC-lower strain had higher toxicity than MC-higher one. Microcystis at different stage of blooms may differentially affect waterfleas owing to variable MaE-induced physiological dysfunction, abundance and grazing rate. Our study suggested that Microcystis strains with lower microcystin-producing ability might release other detrimental chemicals and should not be ignored in harmful bloom monitoring.

Top-down structuring of freshwater bacterial communities by mixotrophic flagellates

Mixotrophic and heterotrophic protists hold a key position in aquatic microbial food webs. Whereas they can account for the bulk of bacterivory in pelagic systems, the potential structuring effect of these consumers on bacterial communities is far from clear. We conducted short-term grazing experiments to test for the overall impact on bacterial community structure and possible prey preferences of phagotrophic protists. The protist taxa selected for this study include three mixotrophic flagellates, comprising two obligate- and one facultative mixotroph, and one phagoheterotrophic flagellate lacking phototrophic capacity. Bacterioplankton from seven different lakes were enriched and used to represent semi-natural prey communities. Our study demonstrated protist strain specific impacts on bacterial community composition linked to grazing. The three mixotrophs had variable impacts on bacterial communities where the two obligate mixotrophs exhibited lower grazing rates, while showing a tendency to promote higher bacterial diversity. The phagoheterotroph displayed the highest grazing rates and structured the bacterial communities via apparent selective grazing. Consistent selectivity trends were observed throughout the experiments, such as the apparent avoidance of all flagellates of Actinobacteria, and high grazing on dominant Burkholderiales taxa. However, there was no consistent “fingerprint” of mixotrophic grazing on prey communities, but the structuring impact rather seemed to depend on the trophic mode of the individual protist taxa, i.e. their dependence on phototrophy vs. phagotrophy. Our findings highlight the differential structuring impact of protist taxa on bacterial communities which may have important ecological implications, for example during periodic dominance of obligate mixotrophic bacterivores in changing lake ecosystems.

How do environmental variables affect the temporal dynamics of zooplankton functional groups in a hyper-eutrophic wetland?

In recent years, trait-based research on zooplankton has gained importance to enable comprehensive interpretation of ecosystem processes, but this approach is still limited in inland waters. Wetlands, one of the most important inland water resources, provide many ecological and economic services in the ecosystem, but like all other water bodies in the world, they are under threat of deterioration and depletion for various reasons. In this study, the taxonomic structure of the zooplankton community in Lake Manyas, one of the important wetlands of Türkiye, as well as their functional characteristics such as body size (small, large) and feeding strategies (microphagous, raptorials) were investigated monthly between 2019 and 2020. The hypothesis that environmental variables influence the species composition and functional groups of zooplankton was tested, given that their structure is shaped by environmental gradients. A total of 62 taxa, including rotifer-dominated, cosmopolite and eutrophication indicator species, were identified. These taxa categorized into three functional groups dominated by small microphagous. Although the zooplankton structure had a high taxonomic diversity, functional homogeneity was observed which reflected a low grazing rate in the ecosystem. Albeit water temperature and total organic carbon significantly affected the distribution of all functional groups, each dominant taxa interacted with different environmental variables. Changing environmental conditions in the lake affected the presence and/or quantity of the zooplankton functional groups (ZFGs). The results showed that ZFGs are strongly associated with environmental conditions and that the response of trait-based functional groups to environmental changes can provide useful knowledge about aquatic ecosystem's health.

Seasonal and spatial comparisons of microzooplankton grazing and phytoplankton growth in the Bohai Bay, China

The dilution experiment technique was used in two cruises in July-August (summer) and October-November (autumn) 2020, with a total of 14 stations. The grazing impact of microzooplankton on phytoplankton in the interior of Bohai Bay was comprehensively investigated. We compared phytoplankton growth rates (μ0) and microzooplankton grazing rates (m) spatially (distance between experimental stations and shore far vs. near) and seasonally (summer vs. autumn). Both m and μ0 values were significantly higher in summer than in autumn, and the phytoplankton growth rate μ0 was positively correlated to temperature. Offshore stations showed higher values. There is no significant spatial and seasonal differences in the ratio of microzooplankton grazing rate and phytoplankton growth rate (m/μ0) indicating that daily consumption of primary production by microzooplankton was similar in the two seasons. Therefore, our research showed a close coupling between microzooplankton grazing with phytoplankton growth in the Bohai Bay.

Urchin grazing of kelp gametophytes in warming oceans.

Sea urchins can cause extensive damage to kelp forests, and their overgrazing can create extensive barren areas, leading to a loss of biodiversity. Barrens may persist when the recruitment of kelp, which occurs through the microscopic haploid gametophyte stage, is suppressed. However, the ecology of kelp gametophytes is poorly understood, and here we investigate if grazing by juvenile urchins on kelp gametophytes can suppress kelp recruitment and if this is exacerbated by climate change. We compared grazing of Ecklonia radiata gametophytes by two species of juvenile urchins, the tropical Tripneustes gratilla and the temperate Centrostephanus rodgersii, atwinter (19°C), summer (23°C), and ocean warming (26°C) temperatures for the low-latitude range edge of E. radiata, which is vulnerable to ocean warming. We examined the rate of recovery of gametophytes following grazing and determined whether they survived and formed sporophytes after ingestion by sea urchins. Both T. gratilla and C. rodgersii grazed E. radiata gametophytes, reducing their abundance compared to no grazing controls. Surprisingly, temperature did not influence grazing rates, but gametophytes did not recover from grazing in the ocean warming (26°C) treatment. Gametophytes survived ingestion by both species of sea urchin and formed sporophytes after ingestion by T. gratilla, but not C. rodgersii. These results suggest complex grazer-gametophyte interactions, in which both negative (reduced abundance and poor recovery with warming) and positive (facilitated recruitment) effects are possible. Small grazers may play a more important role in kelp ecosystem function than previously thought and should be considered in our understanding of alternate stable states.

Disentangling the role of light and nutrient limitation on bacterivory by mixotrophic nanoflagellates.

Many phytoplankton taxa function on multiple trophic levels by combining photosynthesis and ingestion of bacteria, termed mixotrophy. Despite the recognition of mixotrophy as a universal functional trait, we have yet to fully resolve how environmental conditions influence community grazing rates in situ. A microcosm study was used to assess bacterivory by mixotrophic nanoflagellates following nutrient enrichment and light attenuation in a temperate lake. We found contrasting results based on assessment of mixotroph abundance or bacterivory. Despite an interactive effect of nutrient enrichment and light attenuation on mixotroph abundance, significant differences within light treatments were observed only after enrichment with P or N + P. The greatest abundance of mixotrophs across treatments occurred under co-nutrient enrichment with full exposure to irradiance. However, bacterivory by mixotrophic nanoflagellates was greatest under shaded conditions after either N or P enrichment. We suggest that PAR availability dampened the stimulatory effect of nutrient limitation, and bacterivory supplemented a suboptimal photosynthetic environment. In a saturating light regime, the mixotrophic community was less driven to ingest bacteria because photosynthesis was able to satisfy energetic demands. These findings quantify community bacterivory in response to environmental drivers that may characterize future ecosystem conditions and highlight the importance of considering grazing rates in conjunction with abundance of mixotrophic protists.

Grazing rate and feeding selectivity of small and large bodied cladocerans in waters from lakes with different salinity and phytoplankton structure

Grazing rate and feeding selectivity of small and large bodied cladocerans in waters from lakes with different salinity and phytoplankton structure

Using Age Tracers to Estimate Ecological Rates in a Phytoplankton Model

The phytoplankton distribution in estuaries is influenced by multiple spatially variable growth and loss processes. As phytoplankton are transported by tidal and net flows, they are exposed to changing conditions of turbidity, depth, temperature, stratification, and grazing. Understanding the factors influencing the observed phytoplankton distribution patterns will allow better-informed restoration and water management efforts. We developed a Lagrangian approach driven by three-dimensional hydrodynamic model results and a simple representation of the production and losses of phytoplankton, allowing a highly efficient closed-form solution for phytoplankton biomass. Our analysis used continuous observations of chlorophyll concentration at four stations and a near-synoptic chlorophyll dataset collected underway from a boat in the channels of Suisun Marsh in the San Francisco Estuary. We divided the study region into four compartments defined by the water depth and location. For each observation location, hydrodynamic model simulations calculated the time that water parcels spent in each of these compartments and the mean depth encountered by water parcels in those compartments. Then, using that information and continuous monitoring data, we inferred compartment-specific grazing rates and two additional ecological parameters. The underway chlorophyll dataset was used for model validation. The model predicted patterns of observed spatial and tidal variability in chlorophyll in Suisun Marsh. The modeling indicated that the chlorophyll concentration at a point in space in time depends largely on the relative exposure to shallow areas, with positive net productivity and deep areas having negative net productivity.

Escherichia coli Strains Display Varying Susceptibility to Grazing by the Soil Amoeba Dictyostelium discoideum.

Recent studies have shown that Escherichia coli can survive in different environments, including soils, and they can maintain populations in sterile soil for a long period of time. This indicates that growth-supporting nutrients are available; however, when grown in non-sterile soils, populations decline, suggesting that other biological factors play a role in controlling E. coli populations in soil. Free-living protozoa can affect the bacterial population by grazing. We hypothesized that E. coli strains capable of surviving in non-sterile soil possess mechanisms to protect themselves from amoeba predation. We determined the grazing rate of E. coli pasture isolates by using Dictyostelium discoideum. Bacterial suspensions applied to lactose agar as lines were allowed to grow for 24 h, when 4 μL of D. discoideum culture was inoculated in the center of each bacterial line. Grazing distances were measured after 4 days. The genomes of five grazing-susceptible and five grazing-resistant isolates were sequenced and compared. Grazing distance varied among isolates, which indicated that some E. coli are more susceptible to grazing by protozoa than others. When presented with a choice between grazing-susceptible and grazing-resistant isolates, D. discoideum grazed only on the susceptible strain. Grazing susceptibility phenotype did not align with the phylogroup, with both B1 and E strains found in both grazing groups. They also did not align by core genome phylogeny. Whole genome comparisons revealed that the five most highly grazed strains had 389 shared genes not found in the five least grazed strains. Conversely, the five least grazed strains shared 130 unique genes. The results indicate that long-term persistence of E. coli in soil is due at least in part to resistance to grazing by soil amoeba.

Paramecium jenningsi effectively grazes on toxic Raphidiopsis raciborskii and degrades cylindrospermopsin: Implications for control harmful cyanobacterial blooms

Harmful cyanobacterial blooms (HCB) represent a serious threat to aquatic ecosystems due to their production for potent toxins that have negative effects on plant, animal and human health. Although grazing process can be exploited to reduce harmful cyanobacteria in eutrophic waters, the feeding capacity and detoxification capability of the grazer should be considered as key traits of a safe biocontrol agent. In this study, laboratory experiments were conducted to investigate the grazing rate of the ciliate Paramecium jenningsi on toxic Raphidiopsis raciborskii and potential cylindrospermopsin (CYN) degradation during feeding process. P. jenningsi efficiently ingested R. raciborskii cells with high growth (0.1- 0.26 d−1) and grazing rates (0.1-1.4 × 104 cyanobacterial cells ciliate −1 d−1), varying with the initial ciliate density. CYN concentrations within Raphidiopsis cells displayed an elevation under grazing conditions (5.3-7.2 pg cell−1) compared to control cultures (4.4 pg cell−1). CYN concentrations within the ciliate cells showed a reduction (degradation rate =121-276 pg ciliate−1 d−1) and became undetectable after all Raphidiopsis cells were grazed out, indicating that this ciliate could not accumulate CYN within its body during feeding process. Therefore, P. jenningsi could be used as a safe bioagent for the biocontrol of HCBs in water sources. However, a further in situ study is needed for a possible application of Paramecium species for mitigation of toxic cyanobacteria and their cyanotoxins in the natural environment.

The dynamical behavior of a class of stochastic vegetation models

We investigate the influence of random fluctuations in environmental parameters (e.g. rainfall and grazing rate) on the dynamical behavior of a class of ecological models for vegetation biomass in semi-arid regions. Firstly, we show the global existence and uniqueness of positive solutions for the number of vegetation biomass B(t). Then some sufficient conditions are presented for extinction and persistence of B(t) by using the stochastic linearization technique, the Hurwitz criterion and the Lyapunov function method. In particular, a threshold value R0s for the stochastic vegetation model is established in order to illustrate extinction and persistence of B(t). We also find that R0s=1 can be a warning signal for the emergence of desertification. In the case of R0s>1, we show that the stochastic vegetation system has a unique stationary distribution. Finally, we present conditions for persistence of B(t), and prove the pth moment stability and almost sure stability of the null solution in a general decay rate sense (including the exponential, polynomial and logarithmic rates) where the parameters in the stochastic vegetation system can also be dependent on time. These results are illustrated by numerical simulations.

Drivers of spatial and temporal micro- and mesozooplankton dynamics in an estuary under strong anthropogenic influences (The Eastern Scheldt, Netherlands)

The Eastern Scheldt estuary in the Netherlands has been anthropogenically changed by the construction of a storm surge barrier about 30 years ago, affecting abiotic conditions as well as phytoplankton and zooplankton (ZP). As the ZP communities have not been sampled in the last decades, it is unclear to what extent they have changed during the last 30 years. We analyzed the micro- and mesoZP composition and background parameters at eight stations in the Eastern Scheldt in spring, summer, and autumn 2018. Additionally, we conducted dilution experiments to estimate phytoplankton growth and microZP grazing rates. Seasonal variability of ZP was higher than spatial variability, but there were differences between stations. In agreement with data from the 1980s, we identified salinity and chlorophyll a as the most important factors in explaining the ZP community. Suspended particulate matter and water retention time were additionally identified for mesoZP, as well as dissolved inorganic nitrogen and phosphate concentration for microZP. We observed some changes in ZP composition with respect to the 1980s, most of which could be explained by the changes induced by the barrier construction. However, increased abundances of the cyclopoid copepod Oithona sp. were likely related to changes in phytoplankton composition due to large-scale bivalve cultivation in the estuary. Phytoplankton growth and microZP grazing rates were variable between seasons, which could be explained by changes in the phytoplankton community during the annual plankton succession. Overall, the observed alterations in the ZP community due to changed background parameters and intense bivalve filtration could affect the microbial loop as well as consumers of mesoZP and thus the functioning of the pelagic food web.

The Effect of Zooplankton on the Distributions of Dimethyl Sulfide and Dimethylsulfoniopropionate in the Bohai and Yellow Seas

AbstractDimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are ubiquitous sulfur compounds in the ocean. DMS is emitted into the atmosphere and has potential climatic effects. The distributions of DMS and DMSP are affected by various biological factors (i.e., bacterial catabolism and phytoplankton and zooplankton community composition). The horizontal and vertical distributions of DMSP, DMSP lyase activity (DLA), DMS, and the abundances of bacteria, DMSP‐consuming bacteria, dimethyl sulfoxide‐consuming bacteria, and picophytoplankton were investigated in the Bohai Sea (BS) and Yellow Sea (YS) during autumn 2020. DLA was significantly correlated with chlorophyll a, DMS, and dissolved DMSP concentrations. Our data show that bacteria Clade_I SAR11 was a significant contributor to DLA. A dilution experiment indicated that the highest microzooplankton grazing rate coincided with the highest DMS concentration and DMS production rate. A proportion of 16%–62% of the DMSPt was converted to DMS in the dilution experiment. Copepods dominated the mesozooplankton community. Calanus sinicus was the predominant copepod in the BS and YS. C. sinicus grazing stimulated DMS production. DMS concentration increased 299% after C. sinicus grazing on physically broken algal cells for 48 hr. These results will help with a better understanding of the control of DMS and DMSP concentrations by zooplankton and the DMS release mechanisms that occur through zooplankton grazing.

Investigating Factors Contributing to Phytoplankton Biomass Declines in the Lower Sacramento River

Phytoplankton subsidies from river inputs and wetland habitats can be important food sources for pelagic organisms in the Sacramento–San Joaquin Delta (Delta). However, while the Sacramento River is a key contributor of water to the Delta, providing 80% of the mean annual inflow, the river is only a minor source of phytoplankton to the system. The reason for low phytoplankton biomass in the Sacramento River is not well understood but appears to be associated with a 65- km stretch of the lower river where chlorophyll-a (Chl-a) concentrations can decline by as much as 90%. We conducted two surveys along the lower Sacramento River, in spring and fall of 2016, to investigate the relative contributions of different factors potentially driving this Chl-a decline. Our study evaluated the change in Chl-a concentrations as a result of dilution from tributaries, light availability, nutrient concentrations, nutrient uptake, phytoplankton productivity, zooplankton grazing, and clam grazing. Chl-a concentration decreased from 14 µg L–1 to 1.8 µg L–1 in the spring and from 4.0 µg L–1 to 1.2 µg L–1 in the fall. Dilutions from the Feather River and American River contributed to 39% and 11% of Chl-a decline, respectively, during the spring. Average water depths roughly doubled downstream of the American River confluence, reducing water column light availability and lowering productivity. Zooplankton and clam grazing rates were generally low. Using a mass balance analysis, the measured variables explained 76% of the observed decline in Chl-a in the spring, suggesting additional losses from unidentified factors. We found that phytoplankton biomass is regulated by multiple potential factors in the lower Sacramento River, emphasizing the need for practitioners of restoration and management programs to evaluate multiple potential factors when attempting to enhance phytoplankton production in the Delta, or other large river systems.

Carbon and nitrogen uptake through photosynthesis and feeding by photosymbiotic Acantharia

Background: Mixotrophy, combining phagotrophy and photoautotrophy in order to acquire nutrients and energy, is a widespread trophic mode in marine protist plankton. Acantharia (Radiolaria) are ubiquitous, but still uncultured oceanic protists. Many of them are mixoplanktic by endosymbiotic relations with microalgae. Here we aimed at quantitatively assessing phototrophy (inorganic nutrients) and phagotrophy (organic nutrients) of photosymbiotic Acantharia, to understand their physiology, and thereby improve integrations of mixotrophy into ecological models of oceanic ecosystems. Methods: Freshly collected Acantharia were incubated with stable isotopes of inorganic carbon and nitrogen to determine photosynthetic uptake rates. Grazing experiments (prey disappearance) were done with different algal cultures as potential food organisms to measure the contribution of prey ingestion to the acantharian metabolism. Fluorescently (and isotopically) labelled prey was used to verify prey uptake, qualitatively. Results/Conclusions: Carbon uptake rates were unaffected by the nitrogen source (i.e., nitrate or ammonium). Total carbon inorganic uptake rate was 1112±82 pgC h-1 Acantharia‑1, 22.3±1.6 pgC h-1 symbiont cell-1assuming 50 symbionts per Acantharia, at ~155-μmol photons m−2 s−1 irradiance. The Acantharia studied could use both inorganic ammonium and nitrate, but ammonium was taken up at a ~5 times higher rate. Prey ingestion of the haptophyte, Isochrysis galbana, was detected using labelled algae. Significant grazing by Acantharia could only be established on the dinoflagellate Effrenium voratum, with a grazing rate of 728 prey Acantharia‑1 hour-1 (i.e., ~56.3 ngC h-1, 46% of total holobiont carbon content) at a ratio of 1.06x104 prey predator-1. Daily photosynthetic carbon uptake rates made up ~14.5% of the total holobiont carbon content (0.9% hourly). The extent to which photosynthates are used and assimilated by the acantharian cell and/or if it is used for catabolic processes to obtain energy is still to be studied. Isotopic ratios further suggests seasonal differences in the usage of each trophic mode.