Decline In Phytoplankton Biomass Research Articles

Changes in phytoplankton composition hinder the recovery from eutrophication in a perialpine lake (Lake Lugano, Switzerland and Italy)

AbstractWe investigated the factors shaping the response of summer phytoplankton biomass to declining phosphorus (P) concentrations in a lake undergoing restoration (South basin of Lake Lugano, Switzerland and Italy). During 1989–2019, summer P concentrations declined from values typical of eutrophic lakes (>30 μg L−1) to values typical of mesotrophic lakes (10–30 μg L−1). Contrary to expectations, this decline was not followed by a decline in phytoplankton biomass. Instead, phytoplankton biomass showed the highest values in summers with lowest P concentrations. This paradoxical effect was associated with a change in phytoplankton composition. Higher P concentrations were associated with higher relative biomass of green algae, lower P concentrations with higher biomass of cyanobacteria and diatoms. We interpreted this change as a shift from edible and P-demanding phytoplankton to inedible and P-efficient phytoplankton, leading to different trophic structures. The pattern observed may be prompted by the particular conditions of deep lakes approaching mesotrophy, including occurrence of deep-water P reserves and high N concentrations, which can benefit inedible metalimnetic cyanobacteria. To attain reductions in summer phytoplankton biomass, restoration efforts may need to further reduce P concentrations and, possibly, address N enrichment.

Capture of decline in spring phytoplankton biomass derived from COVID-19 lockdown effect in the Yellow Sea offshore waters

The Yellow Sea, characterized as a high-productivity ecosystem, is considered to be significantly attributable to high nutrient supply via atmospheric deposition. We observed a significant decline in phytoplankton biomass (~30%) over the Yellow Sea during February–May 2020 (period of COVID-19 lockdown effect) compared to the same period in 2015–2019 (period of no effect of COVID-19 lockdown). Several possible factors, such as variations in irradiance, vertical mixing, and river discharges, were not major contributors. Through the analysis of transportation and the constituents of atmospheric pollutants from Northern China (main source regions) to the Yellow Sea, we suggest that the decline in phytoplankton biomass over the Yellow Sea is mainly attributed to decreased atmospheric nutrient deposition due to the COVID-19 lockdown effect, because of decreased anthropogenic emissions in Northern China. Thus, attention should be focused on the Yellow Sea ecosystem response to increasing anthropogenic activities by lifting the COVID-19 lockdown restrictions.

Carbon dioxide fluxes from mariculture ponds with swimming crabs and shrimps in eastern China: The effect of adding razor clams

Razor clams (Sinonovacula constricta) are one of the major polyculture species in mariculture ponds, along with swimming crabs (Portunus trituberculatus), in the coastal zones of China, and their effect on CO2 fluxes are poorly studied. CO2 fluxes from mariculture ponds with swimming crabs, kuruma shrimp (Marsupenaeus japonicus) (CS) and mariculture ponds with swimming crabs, kuruma shrimp and razor clams (with different clam densities from low to high, CSB1, CSB2, CSB3, respectively) were measured during the farming season. The CO2 fluxes of CS, CSB1, CSB2, and CSB3 were −25.37, −30.23, −28.81 and −12.18 mg m−2 h−1, respectively. All the ponds acted as CO2 sinks across the water-air interface. Our results showed that water pH was a good indicator of the CO2 flux, and a pH value of 8.24 was the critical value dividing the dataset into influxes or effluxes in the mariculture ponds. Net primary production was also significantly correlated with the CO2 flux and is considered a key driver regulating the CO2 flux. Our findings indicate that the low clam stocking density used in this study could enhance the CO2 sink across the water-air interface by promoting phytoplankton biomass and net primary production, whereas the CO2 sink may be weakened with an increase in clam density due to a decline in phytoplankton biomass and net primary production.

Exploring the Roles of Iron and Irradiance in Dynamics of Diatoms and Phaeocystis in the Amundsen Sea Continental Shelf Water

AbstractThe Amundsen Sea continental shelf (ACS) water ecosystem is expected to undergo changes since increasing melt rate of glacier and decreasing sea ice extent by global warming would lead to the mitigation of iron and light limitation. We investigated how diatoms and Phaeocystis, two dominant taxa, and primary production in the ACS water would respond to variations in iron and light availabilities by using a 1‐D pelagic ecosystem model. In the model, we added sea ice effects that reduce light penetration and optimized model parameters for diatoms and Phaeocystis. The results from our model showed good agreement with 20‐year observations of Chl‐a as well as the biomass proportion of diatoms and Phaeocystis and nutrient distributions during the growing season. Our model experimental results suggest that the current moderate iron and high light conditions favor the growth of Phaeocystis over diatoms. Moreover, as iron increases, the organic carbon exudation by phytoplankton increases more rapidly than net primary production (NPP), leading to a decline in phytoplankton biomass. On the other hand, irradiance plays a role in controlling NPP in terms of photoinhibition which is reduced by increasing iron. Increases in both iron and irradiance lead to an advance in the timing of the bloom peak (surface Chl‐a maximum) due to increases in phytoplankton carbon loss and photoinhibition. Our results imply that the dominance of Phaeocystis can continue and that the carbon uptake capacity of the ACS in the summer seasons might increase given that iron availability will increase with future climate change.

Decline in Phytoplankton Biomass along Indian Coastal Waters due to COVID-19 Lockdown

The SARS-CoV-2 (or COVID-19) lockdown in India, which started at an early stage of its infection curve, has been one of the strictest in the world. Air quality has improved in all urban centers in India, a major emitter of greenhouse gases (GHG). This study is based on the hypothesis that an abrupt halt in all urban activities resulted in a massive decline in NO2 emissions and has also altered coastal nitrogen (N) inputs; in-turn, this affected the trophic status of coastal waters across the country. We present the first evidence of an overall decline in pre-monsoon chlorophyll-a, a proxy for phytoplankton biomass, in coastal waters off urban centers during the peak of the lockdown in April. The preliminary field data and indirect evidence suggests the reduction in coastal chlorophyll-a could be linked to a net decline in nutrient loading, particularly of bioavailable N through watershed fluxes and atmospheric deposition. The preliminary results stress the importance of a further understanding of the relationship between fluctuations in anthropogenic N, due to lockdown measures and coastal ecosystem responses, as countries open-up to a business-as-usual scenario.

Partitioning the influence of hydrodynamics-induced physical variables and nutrients on phytoplankton assemblages in a shallow tropical reservoir (Koka, Ethiopia)

Ecological processes, population dynamics and species interactions in shallow lakes and reservoirs are affected by seasonal and inter-annual fluctuations in rainfall and runoff. The present study aimed to explore the relationship between riverine runoff and phytoplankton community structure in Koka Reservoir, Ethiopia. Onsite measurements and collection of water samples and phytoplankton were performed at monthly intervals from January to August 2016 at three sampling sites along a stretch of the reservoir running from the river mouth to center and the outlet (dam). The results showed the decline in phytoplankton biomass at the river mouth site. The decline was particularly noticeable during the period of high rainfall and runoff compared to that of the low water period. This was associated with the hydrodynamics-induced high turbidity and reduced euphotic depth. Our results implied that phytoplankton community structure was strongly influenced by flood pulse. The results suggest that river flow and watershed management have the potential to be used more widely as an aquatic resources management option in small and medium-sized lakes and reservoirs.

Low flows and downstream decline in phytoplankton contribute to impaired water quality in the lower Minnesota River

The underlying physical and biogeochemical mechanisms associated with low dissolved oxygen (DO) levels below 5 mg L−1 were examined through field data analyses and water quality modeling of the lower 40 miles of the Minnesota River. Insights into flow and water quality data of nineteen years (1999–2017) at five sites demonstrate that low DO levels parallel the obvious longitudinal (upstream-to-downstream) decline in phytoplankton biomass and increase in ammonium nitrogen (NH4) and dissolved orthophosphate (PO4) in the last 22-mile river reach (i.e., navigation channel) during late summer low flow conditions. River discharge is inversely related to the magnitude of the longitudinal change in DO, phytoplankton biomass, NH4 and PO4, indicating that the late summer low flow hydrodynamics in the navigation channel with a longer residence time, deeper water and slower velocity provide an extended opportunity for the biogeochemical reactions involving phytoplankton, DO and nutrients. Moreover, the ratio of the longitudinal decline in DO versus the longitudinal increase in NH4 is particularly close to the Redfield O:N ratio, suggesting that the decline in DO and increase in nutrients most likely result from the decomposition of phytoplankton detritus under aerobic conditions. This is further proved by the water quality modeling of the lower Minnesota River. The primary reasons for impaired water quality are substantially elevated sediment oxygen consumption and nutrient release derived from the decomposition of settled phytoplankton detritus in the navigation channel. Therefore, we recommend that active prevention of abrupt phytoplankton blooms and collapses through regulation of river discharge and local hydrodynamics may assist in maintaining acceptable water quality in eutrophic rivers with a high level of phytoplankton biomass.

First steps of ecological restoration in Mediterranean lagoons: Shifts in phytoplankton communities

Along the French Mediterranean coast, a complex of eight lagoons underwent intensive eutrophication over four decades, mainly related to nutrient over-enrichment from continuous sewage discharges. The lagoon complex displayed a wide trophic gradient from mesotrophy to hypertrophy and primary production was dominated by phytoplankton communities. In 2005, the implementation of an 11 km offshore outfall system diverted the treated sewage effluents leading to a drastic reduction of anthropogenic inputs of nitrogen and phosphorus into the lagoons. Time series data have been examined from 2000 to 2013 for physical, chemical and biological (phytoplankton) variables of the water column during the summer period. Since 2006, total nitrogen and phosphorus concentrations as well as chlorophyll biomass strongly decreased revealing an improvement in lagoon water quality. In summertime, the decline in phytoplankton biomass was accompanied by shifts in community structure and composition that could be explained by adopting a functional approach by considering the common functional traits of the main algal groups. These phytoplankton communities were dominated by functional groups of small-sized and fast-growing algae (diatoms, cryptophytes and green algae). The trajectories of summer phytoplankton communities displayed a complex response to changing nutrient loads over time. While diatoms were the major group in 2006 in all the lagoons, the summer phytoplankton composition in hypertrophic lagoons has shifted towards green algae, which are particularly well adapted to summertime conditions. All lagoons showed increasing proportion and occurrence of peridinin-rich dinophytes over time, probably related to their capacity for mixotrophy. The diversity patterns were marked by a strong variability in eutrophic and hypertrophic lagoons whereas phytoplankton community structure reached the highest diversity and stability in mesotrophic lagoons. We observe that during the re-oligotrophication process in coastal lagoons, phytoplankton shows complex trajectories with similarities with those observed in freshwater lake systems.

Warming, but not enhanced CO2 concentration, quantitatively and qualitatively affects phytoplankton biomass

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 528:39-51 (2015) - DOI: https://doi.org/10.3354/meps11264 Warming, but not enhanced CO2 concentration, quantitatively and qualitatively affects phytoplankton biomass Carolin Paul*, Birte Matthiessen, Ulrich Sommer GEOMAR Helmholtz-Centre for Ocean Research, Düsternbrooker Weg 20, 24105 Kiel, Germany *Corresponding author: cpaul@geomar.de ABSTRACT: We investigated the impacts of predicted ocean acidification and future warming on the quantity and nutritional quality of a natural phytoplankton autumn bloom in a mesocosm experiment. Since the effects of CO2-enrichment and temperature have usually been studied independently, we were also interested in the interactive effects of both aspects of climate change. Therefore, we used a factorial design with 2 temperature and 2 acidification levels in a mesocosm experiment with a Baltic Sea phytoplankton community. Our results show a significant time-dependent influence of warming on phytoplankton carbon, chlorophyll a, and particulate organic carbon. Phytoplankton carbon, for instance, decreased by more than half with increasing temperature at bloom time. Additionally, elemental carbon to phosphorus ratios (C:P) increased significantly, by approximately 5 to 8%, due to warming. Impacts of CO2 or synergetic effects of warming and acidification could not be detected. We suggest that stronger grazing pressure induced by temperature was responsible for the significant decline in phytoplankton biomass. Our results suggest that the biological effects of warming on Baltic Sea phytoplankton are considerable and will likely have fundamental consequences for trophic transfer in the pelagic food web. KEY WORDS: Phytoplankton biomass · Warming · Ocean acidification Full text in pdf format Supplementary material PreviousNextCite this article as: Paul C, Matthiessen B, Sommer U (2015) Warming, but not enhanced CO2 concentration, quantitatively and qualitatively affects phytoplankton biomass. Mar Ecol Prog Ser 528:39-51. https://doi.org/10.3354/meps11264 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 528. Online publication date: May 28, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

An unusually large phytoplankton spring bloom drives rapid changes in benthic diversity and ecosystem function

In 2012, the Western English Channel experienced an unusually large and long-lived phytoplankton spring bloom. When compared with data from the past 20years, average phytoplankton biomass at Station L4 (part of the Western Channel Observatory) was approximately 3× greater and lasted 50% longer than any previous year. Regular (mostly weekly) box core samples were collected from this site before, during and after the bloom to determine its impact on macrofaunal abundance, diversity, biomass, community structure and function. The spring bloom of 2012 was shown to support a large and rapid response in the majority of benthic taxa and functional groups. However, key differences in the precise nature of this response, as well as in its timing, was observed between different macrofauna feeding groups. Deposit feeders responded almost instantly at the start of the bloom, primarily thorough an increase in abundance. Suspension feeders and opportunistic/predatory/carnivorous taxa responded slightly more slowly and primarily with an increase in biomass. At the end of the bloom a rapid decline in macrobenthic abundance, diversity and biomass closely followed the decline in phytoplankton biomass. With suspension feeders showing evidence of this decline a few weeks before deposit feeders, it was concluded that this collapse in benthic communities was driven primarily by food availability and competition. However, it is possible that environmental hypoxia and the presence of toxic benthic cyanobacteria could also have contributed to this decline. This study shows evidence for strong benthic–pelagic coupling at L4; a shallow (50m), coastal, fine-sand habitat. It also demonstrates that in such habitats, it is not just planktonic organisms that demonstrate clear community phenology. Different functional groups within the benthic assemblage will respond to the spring bloom in specific manner, with implications for key ecosystem functions and processes, such as secondary production and bioturbation. Only by taking integrated benthic and pelagic observations over such fine temporal scales (weekly) was the current study able to identify the intimate structure of the benthic response. Similar studies from other habitats and under different bloom conditions are urgently needed to fully appreciate the strength of benthic–pelagic coupling in shallow coastal environments.

Seasonal dynamics of physical and biological processes in the central California Current System: A modeling study

A 3-D physical and biological model is used to study the seasonal dynamics of physical and biological processes in the central California Current System. Comparisons of model results with remote sensing and in situ observations along CalCOFI Line 67 indicate our model can capture the spatial variations of key variables (temperature, nutrients, chlorophyll, and so on) on annual mean and seasonal cycle. In the coastal upwelling system, it is the alongshore wind stress that upwells high nutrients to surface from 60 m and stimulates enhanced plankton biomass and productivity in the upwelling season. As a result, coastal species peak in the late upwelling period (May–July), and oceanic species reach the annual maxima in the oceanic period (August–October). The annual maximum occurs in the late upwelling period for new production and in the oceanic period for regenerated production. From the late upwelling period to the oceanic period, stratification is intensified while coastal upwelling becomes weaker. Correspondingly, the coastal ecosystem retreats from ∼300 to ∼100 km offshore with significant decline in chlorophyll and primary production, and the oceanic ecosystem moves onshore. During this transition, the decline in phytoplankton biomass is due to the grazing pressure by mesozooplankton in the 0–150 km domain, but is regulated by low growth rates in the 150–500 km offshore domain. Meanwhile, the growth rates of phytoplankton increase in the coastal waters due to deeper light penetration, while the decrease in offshore growth rates is caused by lower nitrate concentrations.

Plankton trends and community changes in a coastal sea (northern Adriatic): Bottom-up vs. top-down control in relation to environmental drivers

The shallow and land-locked northern Adriatic basin has undergone several changes at different trophic levels over the last decades, of which the most recent and basin-wide is the negative trend in concentrations of chlorophyll a over the last decade. To investigate it in detail, we performed a robust analysis of abiotic parameters and plankton community structure in the period 1989–2009 in the Gulf of Trieste. The gradual sequence of events, which encompassed hydrological and ecological systems, was identified as a regime shift in 2002/2003. Synchronised step changes in river runoff and surface salinity and decreasing concentrations of nitrate and silicate were followed by changes in chlorophyll a. The decline in phytoplankton biomass was due to the reduction of seasonal diatom blooms and the predominance of the smaller sized fraction. The changes also encompassed a higher trophic level showing significantly reduced zooplankton biomass after 2002.Our results show that despite the scientifically recognised impact of large-scale climate drivers on Adriatic plankton communities, the Gulf remains largely controlled, either through local climatic (precipitation, river runoff) or anthropogenic (eutrophication) forcing. Under the resource-replete and high biomass regime bottom-up control prevails, while in the new regime of reduced resources and biomass the switch from bottom-up to top-down control can occur seasonally.

The role of ciliates, heterotrophic dinoflagellates and copepods in structuring spring plankton communities at Helgoland Roads, North Sea

Mesocosm experiments coupled with dilution grazing experiments were carried out during the phytoplankton spring bloom 2009. The interactions between phytoplankton, microzooplankton and copepods were investigated using natural plankton communities obtained from Helgoland Roads (54°11.3′N; 7°54.0′E), North Sea. In the absence of mesozooplankton grazers, the microzooplankton rapidly responded to different prey availabilities; this was most pronounced for ciliates such as strombidiids and strobilids. The occurrence of ciliates was strongly dependent on specific prey and abrupt losses in their relative importance with the disappearance of their prey were observed. Thecate and athecate dinoflagellates had a broader food spectrum and slower reaction times compared with ciliates. In general, high microzooplankton potential grazing impacts with an average consumption of 120% of the phytoplankton production (Pp) were measured. Thus, the decline in phytoplankton biomass could be mainly attributed to an intense grazing by microzooplankton. Copepods were less important phytoplankton grazers consuming on average only 47% of Pp. Microzooplankton in turn contributed a substantial part to the copepods’ diets especially with decreasing quality of phytoplankton food due to nutrient limitation over the course of the bloom. Copepod grazing rates exceeded microzooplankton growth, suggesting their strong top-down control potential on microzooplankton in the field. Selective grazing by microzooplankton was an important factor for stabilising a bloom of less-preferred diatom species in our mesocosms with specific species (Thalassiosira spp., Rhizosolenia spp. and Chaetoceros spp.) dominating the bloom. This study demonstrates the importance of microzooplankton grazers for structuring and controlling phytoplankton spring blooms in temperate waters and the important role of copepods as top-down regulators of microzooplankton.

Environmental controls on phytoplankton production in coastal ecosystems: A case study from Tokyo Bay

Phytoplankton biomass and primary production were examined in their environmental context, for a semi-enclosed bay (Tokyo Bay, Japan) using data from monthly samples collected over a three-year period. Heavy precipitation and high surface temperatures in the late spring and summer gave rise to a highly-stratified water-column and stimulated a series of phytoplankton blooms, whereas during the winter, a weakly-stratified and deeply-mixed water-column led to a rapid decline in phytoplankton biomass under light-limited growth conditions. By incorporating pigment, photophysiological and optical data into a primary production model we show that daily, water-column primary production ranges from ∼160 mg C m −2 d −1 to 7600 mg C m −2 d −1. High water turbidity and deep vertical mixing, both separately and in concert, limit the light available for algal growth over much of the year. Annual primary production varied from 370 to 580 g C m −2 y −1. The relative influences of nutrient limitation and light limitation are assessed. A model is developed that describes this in an explicit manner using photophysiological parameters.

Seasonal changes in phytoplankton growth and microzooplankton grazing at an Antarctic coastal site

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 50:157-167 (2008) - DOI: https://doi.org/10.3354/ame01149 Seasonal changes in phytoplankton growth and microzooplankton grazing at an Antarctic coastal site Imojen Pearce, Andrew T. Davidson*, Simon Wright, Rick van den Enden Australian Government Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre, Channel Highway, Kingston, 7050 Tasmania, Australia *Corresponding author. Email: andrew.davidson@aad.gov.au ABSTRACT: Despite being a major pathway for carbon flow in aquatic systems, little is known about annual changes in microzooplankton grazing in Antarctic waters and its top-down control of phytoplankton. We determined changes in the growth and mortality of phytoplankton in near-shore waters off East Antarctica between February and November 2004 using the grazing dilution technique. Results showed large seasonal variation in the microzooplankton grazing rate and top-down control of phytoplankton. In late summer (February to March), microzooplankton consumed around 34% of primary production and 33% of the phytoplankton standing stock d–1. As sea ice formed in early April, grazing mortality increased to 762% of primary production and 72% of the phytoplankton standing stock d–1, coinciding with a rapid decline in phytoplankton biomass, cell volume and chlorophyll a concentration. In winter (late April to September), phytoplankton abundance and productivity were insufficient to sustain extensive herbivory; only 5 of the 12 winter experiments yielded microzooplankton grazing rates that were significantly different from zero. The few significant experiments showed that microheterotrophs occasionally consumed ca. 54% of the phytoplankton standing stock and >100% primary production d–1. In spring (October to November), rates of microzooplankton grazing and phytoplankton growth were highly significant but negative. Our study showed that microzooplankton contributed substantially to the termination of the summer phytoplankton bloom. Thereafter, microzooplankton grazing, though occasionally substantial, was commonly low or insignificant. Furthermore, during the aphotic Antarctic winter, negative rates of phytoplankton growth commonly exceeded those of microzooplankton grazing, suggesting that herbivory was not the principal cause of phytoplankton mortality. KEY WORDS:Antarctic · Microzooplankton · Grazing · Phytoplankton growth Full text in pdf format PreviousNextCite this article as: Pearce I, Davidson AT, Wright S, van den Enden R (2008) Seasonal changes in phytoplankton growth and microzooplankton grazing at an Antarctic coastal site. Aquat Microb Ecol 50:157-167. https://doi.org/10.3354/ame01149 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 50, No. 2. Online publication date: January 31, 2008 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2008 Inter-Research.

Photoacoustics: a novel tool for the determination of photosynthetic energy storage efficiency in phytoplankton

Photosynthetic energy storage efficiency controls the development and decline of phytoplankton biomass. All abiotic environmental factors such as light intensity; temperature, nutrient availability and pollutants will exert detectable changes in the photosynthetic energy storage efficiency of phytoplankton, and subsequently affect total biomass and composition of phytoplankton assemblages. Since this efficiency is a sensitive amplifier of ambient conditions, it thereby is an excellent reporter of water quality parameter. We demonstrate the applicability of the novel photoacoustic method in easily and directly estimating the energy storage efficiency of phytoplankton in a drinking water reservoir of different nutrient status.

Interactions between nutrients, phytoplankton growth, and micro- and mesozooplankton grazing in the plume of the Mississippi River

In March 2002, we conducted 6 shipboard grazing experiments spanning locations from near the largest discharge point of the Mississippi River to a far-field, high salinity location approxi- mately 60 km to the SW. Waters were characterized by salinity, nitrate and size-fractionated chloro- phyll a (chl a). None of our stations were in truly oligotrophic conditions typical of the open Gulf of Mexico. Experiments measured growth of 3 phytoplankton size categories ( 20 µm) and associated microzooplankton and mesozoplankton grazing. Rates of phytoplankton growth in all size categories were high in the near- and intermediate-fields, but declined dramatically in the far- field due to nutrient limitation. Microzooplankton grazing rates were low in the near-field, highest in intermediate stations and then declined in the far-field. The mesozooplankton grazing rate was gen- erally low compared to the microzooplankton grazing rate. The mesozooplankton grazing rate was highest at the far-field station where more than 86% of daily growth of > 20 µm phytoplankton was consumed by mesozooplankton. As the plume dispersed and mixed with higher salinity shelf water, inorganic nutrients became exhausted and phytoplankton growth (especially the large diatoms) became nutrient-limited. During this transition, the microzooplankton grazing rate surpassed the phytoplankton growth rate, causing a decline in phytoplankton biomass. Mesozooplankton grazing enhanced this decline by adding an additional grazing mortality, especially to the large phytoplank- ton cells. The dynamics between growth and grazing mortality in the different size fractions of the phytoplankton community, combined with varying growth rates associated with declining nutrient concentrations, and superimposed on a background of dilution derived from mixing of plume waters with oligotrophic oceanic water, makes interface regions between large rivers and the ocean excep- tionally complex.

Competition for inorganic nutrients between phytoplankton and bacterioplankton in nutrient manipulated mesocosms

A manipulative mesocosm experiment in Danish coastal waters tested the effect on plankton biodiversity and function of adding nitrate, phosphate and glucose. A comprehensive set of measurements was made over a 6 d period; these included phytoplankton biomass and production in 3 size fractions (>10, 10-2 and <2 µm), bacterial biomass and production, nitrate and ammonium uptake, and pigment taxonomy. Addition of nitrate and phosphate resulted in increases of biomass and production of all size fractions of phytoplankton. Inorganic nutrients alone had only a minor effect on bacterial abundance and production, with slight increases relative to the control. The largest changes occurred in mesocosms to which glucose was added in excess with nitrate and phosphate. Pigment composition indicated little change in phytoplankton assemblage composition in any treatment. A large increase in bacterial activity in the presence of added glucose had a negative effect on the phytoplankton assemblage and resulted in a decline in phytoplankton biomass. Data on nutrient uptake and size-fractionated carbon fixation suggest that the mechanism of this phyto- plankton suppression was the ability of heterotrophic bacteria to out-compete for available inorganic nutrients, resulting in nutrient limitation of the phytoplankton assemblage.

Piscivores, trophic cascades, and lake management.

The concept of cascading trophic interactions predicts that an increase in piscivore biomass in lakes will result in decreased planktivorous fish biomass, increased herbivorous zooplankton biomass, and decreased phytoplankton biomass. Though often accepted as a paradigm in the ecological literature and adopted by lake managers as a basis for lake management strategies, the trophic cascading interactions hypothesis has not received the unequivocal support (in the form of rigorous experimental testing) that might be expected of a paradigm. Here we review field experiments and surveys, testing the hypothesis that effects of increasing piscivore biomass will cascade down through the food web yielding a decline in phytoplankton biomass. We found 39 studies in the scientific literature examining piscivore effects on phytoplankton biomass. Of the studies, 22 were confounded by supplemental manipulations (e.g., simultaneous reduction of nutrients or removal of planktivores) and could not be used to assess piscivore effects. Of the 17 nonconfounded studies, most did not find piscivore effects on phytoplankton biomass and therefore did not support the trophic cascading interactions hypothesis. However, the trophic cascading interactions hypothesis also predicts that lake systems containing piscivores will have lower phytoplankton biomass for any given phosphorus concentration. Based on regression analyses of chlorophyll�total phosphorus relationships in the 17 nonconfounded piscivore studies, this aspect of the trophic cascading interactions hypothesis was supported. The slope of the chlorophyll vs. total phosphorus regression was lower in lakes with planktivores and piscivores compared with lakes containing only planktivores but no piscivores. We hypothesize that this slope can be used as an indicator of “functional piscivory” and that communities with extremes of functional piscivory (zero and very high) represent classical 3- and 4-trophic level food webs.

Can a community of small‐bodied grazers control phytoplankton in rivers?

1. Phytoplankton, zooplankton and grazing were monitored throughout the growing season for three years (1994–96) in the Belgian section of the River Meuse.2. A size structure analysis of the algal community shows that there was a summer shift toward larger algal units, following a decline in phytoplankton biomass. These changes occurred after an increase in zooplankton biomass and diversity.3. Daily filtration rates of grazers ranged from 1 to 113% day–1 and maxima were observed during the summer period. Higher rates tended to correspond with peaks of rotifer biomass. A decline in total phytoplankton biomass within two weeks followed the increase in zooplankton biomass and filtration rate. A rapid biomass recovery was then observed, along with a shift of the algal community toward larger units. When grazing activity was not sustained, due to zooplankton fluctuations, the change in phytoplankton size structure was less marked.4. We suggest that the composition of the phytoplankton community of large rivers may at times be controlled by grazers. However, such biotic interactions can take place only when physical constraints are reduced, i.e. when discharge is low, and when increased transfer time, high temperature and availability of grazeable algae allow high zooplankton biomass.