Top-down Control Of Phytoplankton Research Articles

Pathways linking nutrient enrichment, habitat structure, and parasitism to host-resource interactions.

Human activities simultaneously alter nutrient levels, habitat structure, and levels of parasitism. These activities likely have individual and joint impacts on food webs. Furthermore, there is particular concern that nutrient additions and changes to habitat structure might exacerbate the size of epidemics and impacts on host density. We used a well-studied zooplankton-fungus host-parasite system and experimental whole water column enclosures to factorially manipulate nutrient levels, habitat structure (specifically: mixing), and presence of parasites. Nutrient addition increased infection prevalence, density of infected hosts, and total host density. We hypothesized that nutrients, mixing, and parasitism were linked in multiple ways, including via their combined effects on phytoplankton (resource) abundance, and we used structural equation modeling to disentangle these pathways. In the absence of the parasite, both nutrients and mixing increased abundance of phytoplankton, whereas host density was negatively related to phytoplankton abundance, suggesting a mixture of bottom-up and top-down control of phytoplankton. In the presence of the parasite, nutrients still increased phytoplankton abundance but mixing no longer did, and there was no longer a significant relationship between host density and phytoplankton. This decoupling of host-resource dynamics may have resulted from reduced grazing due to illness-mediated changes in feeding behavior. Overall, our results show that the impact of one human activity (e.g., altered habitat structure) might depend on other human impacts (e.g., parasite introduction). Fortunately, carefully designed experiments and analyses can help tease apart these multifaceted relationships, allowing us to understand how human activities alter food webs, including interactions between hosts and their parasites and resources.

Bottom-up and top-down effects on phytoplankton functional groups in Hulun Lake, China

The debates about the extent to which phytoplankton in freshwater ecosystems are regulated by top-down or bottom-up forces have been ongoing for decades. This study examines the effects of bottom-up and top-down factors on the phytoplankton functional groups in a eutrophic lake. Phytoplankton and zooplankton were sampled and physical-chemical variables measured from May 2019 to October 2019 in Lake Hulun, China. Approximately 43 phytoplankton species were observed and grouped into 23 functional groups. For the zooplankton, about 27 species were observed and classified into 8 functional groups. The study revealed that the bottom-up effects of physical-chemical variables on some phytoplankton functional groups was stronger than the top-down effects of zooplankton. Water temperature (WT), total phosphorus (TP), total nitrogen (TN), conductivity (Cond), water transparency (SD), and dissolved oxygen (DO) significant influence the biomass of the phytoplankton functional groups. The biomass of phytoplankton functional groups was influenced positively by nutrient availability likely because nutrients influence the growth and reproduction of phytoplankton in freshwater. WT and DO had a positive influence on biomass of phytoplankton functional groups. Conversely, phytoplankton biomass revealed a decreasing trend when SD and Cond significantly increased. This study showed that zooplankton functional groups were positively correlated with phytoplankton biomass implying that the top-down control of phytoplankton by the zooplankton in the lake is not strong enough to produce a negative effect. It is evident that the zooplankton functional groups in Lake Hulun are controlled more by bottom-up force than top-down.

Zooplankton abundance and community structure driven by tidal currents in a Mediterranean coastal lagoon (Boughrara, Tunisia, SW Mediterranean Sea)

Lagoons and coastal marine zones are very productive and useful ecosystems, but they are threatened by the effects of global change and anthropogenic pressures. These effects have a negative impact on the zooplankton, weakening its function of phytoplankton consumer, leading to uncontrolled proliferation of microalgae in case of eutrophication. In this study we test the hypothesis that tidal exchanges with the sea can counteract these deleterious effect, by renewing the zooplankton community and by enhancing its top-down control of phytoplankton through selective retention of zooplankton grazers. Our study focused on the southern region of the Gulf of Gabes and the Boughrara lagoon which presents the highest tidal range in the Mediterranean. During two field campaigns (October 2016 and April 2017) we have analyzed zooplankton descriptive (taxonomic composition, abundance; biomass) and functional (ingestion rate, grazing pressure) variables and environmental variables during time series at a fixed station during ebb-flood sequences and at 8 stations along a sea-lagoon transect and during different tidal amplitude periods. Multivariate analyses of both environmental parameters and zooplankton taxa showed the existence of three distinct zones along the sea-lagoon transect, but also the influence of tidal circulation and water mixing on the renewal of the zooplankton community up to the innermost zones of the lagoon. Time series gave clear patterns for the input/output of marine/lagoon species and show a net import for different taxa and for the total zooplankton abundance and biomass (mean flood/ebb ratio = 2.2 for the total abundance and 2.4 for the biomass), leading to differential retention of zooplankton in the lagoon and to an increase in the potential grazing pressure on phytoplankton (mean flood/ebb ratio = 2.8). We also estimated that the grazing pressure in the lagoon was twice higher during the periods of high tidal amplitudes (at or close to spring tide) compared to periods with low amplitude (neap tides), clearly showing that the retention of zooplankton significantly increases the grazing impact on phytoplankton. These results highlight the importance of the tidal forcing for maintaining the good status of the zooplankton structure and function in strongly anthropized coastal and lagoon ecosystems.

Seasonal dynamics of Zooplankton functional groups in relation to environmental variables in Xiquanyan Reservoir, Northeast China

In this study, the concept of functional feeding groups was used to classify and model the seasonal variation of zooplankton functional groups in relation to environmental variables. A total of 48 zooplankton species were observed in the reservoir and grouped into 8 functional groups. Both environmental variable and the biomass of zooplankton functional groups vary spatially and seasonally. Water temperature, water transparency, total nitrogen and nitrates were significantly higher in summer, while chlorophyll-aand ammonium were higher in autumn and spring, respectively. Biomass of zooplankton was significantly higher in summer (245.81 μg/L), followed by autumn (196.54 μg/L) and spring (54.25 μg/L). Group RF (rotifer filter feeders) dominated in spring, accounting for 80% of the total biomass. In summer and autumn, group RC (rotifer carnivore) and SCF (small copepods and cladocerans filter feeders) were the dominant, respectively. Total nitrogen, total phosphorus, ammonium, chlorophyll-aand water transparency were the major factor influencing zooplankton community. Group RF was positively influenced by ammonium and total phosphorus, while RC, SCF and MCF (middle copepods and cladocerans filter feeders) were positively correlated with chlorophyll-a. Top-down control of phytoplankton by groups RC, SCF and MCF in Xiquanyan reservoir is not strong enough to produce negative effect. Increase in predator size biomass did not strengthen top-down control on prey. It is quite clear that the zooplankton function groups of Xiquanyan reservoir followed a predictable seasonal pattern. This therefore highlights the significance of environmental variables in structuring plankton composition in the reservoir.

Limnology of the neotropical high elevation shallow lake Yahuarcocha (Ecuador) and challenges for managing eutrophication using biomanipulation

Temperate and tropical shallow lakes differ in several fundamental aspects with respect to management of eutrophication. High altitude tropical shallow lakes are a special case, showing similarities with temperate and tropical lakes. We studied the ecology of the eutrophic high-altitude tropical lake Yahuarcocha in the Ecuadorian Andes and evaluated the potential of biomanipulation to control eutrophication. With a toxin-producing Cylindrospermopsis bloom, low Secchi depth and low submerged macrophyte cover, Yahuarcocha is clearly in a turbid ecosystem state. Relatively low nutrient concentrations should theoretically allow for a shift to a clear water state through biomanipulation. Top-down control of phytoplankton by zooplankton, however, is complicated by the (1) absence of predatory fish, (2) fish community dominated by small poecelid species, (3) lack of a refuge for zooplankton from fish predation within the macrophytes, and (4) persistent, grazing resistant bloom of the cyanobacterium Cylindrospermopsis. In these aspects, lake Yahuarcocha is more similar to tropical shallow lakes, probably because water temperature is high relative to the mean air temperature and because of the absence of a cold season. The fish and macrophyte communities consisted almost entirely of exotic species. The exotic fish species probably stabilized the turbid state in the lake.

Copepod grazing and their impact on phytoplankton standing stock and production in a tropical coastal water during the different seasons

The grazing rate of copepods on the total and size-fractionated phytoplankton biomass in a coastal environment (off Kochi, southwest coast of India) were measured during pre-monsoon (PRM), peak southwest monsoon (PKSWM), late southwest monsoon (LSWM) and post-southwest monsoon (PSWM). The phytoplankton standing stock (chlorophyll a-Chl. a) and growth rate (GR) were less during the PRM (Chl. a 0.58mgm-3; GR 0.23±0.02) and PSWM (Chl. a 0.89mgm-3; GR 0.30±0.05) compared to PKSWM (Chl. a 6.67mgm-3; GR 0.43±0.02) and LSWM (Chl. a 4.09mgm-3; GR 0.40±0.04). The microplankton contribution to the total Chl. a was significant during the PKSWM (41.83%) and LSWM (45.72%). Copepod density was lesser during the PRM (1354Nom-3) and PSWM (1606Nom-3) than during PKSWM and LSWM (4571 and 3432Nom-3, respectively). Seasonal changes in phytoplankton biomass, phytoplankton size structure, and copepod community were closely related to the hydrographical transformations in the study domain. Dominant calanoid copepods in the study region ingested 8.4 to 14.2% of their daily ration from phytoplankton during the PRM and PSWM, which increased to >50% during the PKSWM and LSWM. The cyclopoid Oithona similis was abundant during the PKSWM, ingesting only 21% of their daily ration from phytoplankton. Temporal variation in the phytoplankton biomass and copepod species composition caused differences in community level top-down control. The copepod community ingestion on phytoplankton was high during the LSWM (18,583μgCm-3d-1), followed by PKSWM (9050μgCm-3d-1), PSWM (1813μgCm-3d-1), and PRM (946μgCm-3d-1). During the low Chl. a period (PRM and PSWM), dominant calanoid copepods showed a positive selectivity for the micro- and nano-phytoplankton size fractions, whereas during the high Chl. a period (PKSWM and LSWM), they showed a positive selection for nano-phytoplankton fractions. Irrespective of the seasons, dominant calanoid copepods showed a negative selection of pico-phytoplankton fraction. The cyclopoid O. similis and Poecilostomatoid Corycaeus danae showed a positive selection of nano- and pico-phytoplankton fractions rather than micro-fraction. The grazing pressure of copepod community ingestion on micro-fraction was less (0.56% of the phytoplankton biomass and 1.06% of the phytoplankton production) during the PKSWM. This study provides, for the first time, clear findings on the seasonal variation in the top-down control of phytoplankton by copepods in a tropical coastal water ecosystem and discusses its implications on phytoplankton blooming, plankton food web, and biogeochemistry.

Is fish able to regulate filamentous blue-green dominated phytoplankton?

Efficient zooplankton grazing is a prerequisite for establishing a cascading food web control over phytoplankton in a lake. We studied if the top-down impact of fish could reach phytoplankton in a lake where the grazing pressure of small-sized zooplankton on filamentous phytoplankton is considered weak. We analysed >30 years of data on plankton, fish catches, hydrochemistry, hydrology, and meteorology from Vortsjarv, a large and shallow eutrophic lake in Estonia with intensive commercial fisheries. The lake’s unregulated water level has been considered the strongest factor affecting the ecosystem through modifying sediment resuspension, internal loading of nutrients, and underwater light conditions and spawning conditions for fish. We found a negative relationship between phytoplankton biomass and pikeperch biomass indicating a potential top-down cascading effect in the food web. Top-down control of phytoplankton by zooplankton was reflected in a negative relationship between phyto- and zooplankton biomasses. A decrease of the individual weight of crustacean zooplankton with increasing biomass of small fish suggested top-down control of zooplankton by planktivorous fish. In contrast, we could not demonstrate a direct linkage between piscivorous fish and small fish. The top-down food web impact of piscivores, however, was manifested at zooplankton level in a positive correlation of pikeperch biomass with the biomass of dominating cladoceran species Bosmina coregoni and the individual weight of copepods. At high biomasses of small fish, ciliate domination over metazooplankton increased and thus enhanced the strength of the microbial food web. According to our results, fishery management measures that increase small plankti- and benthivorous fish biomass have to be avoided as they have a cascading negative effect on the ecosystem health.

Warmer, deeper, and greener mixed layers in the North Atlantic subpolar gyre over the last 50 years.

Shifts in global climate resonate in plankton dynamics, biogeochemical cycles, and marine food webs. We studied these linkages in the North Atlantic subpolar gyre (NASG), which hosts extensive phytoplankton blooms. We show that phytoplankton abundance increased since the 1960s in parallel to a deepening of the mixed layer and a strengthening of winds and heat losses from the ocean, as driven by the low frequency of the North Atlantic Oscillation (NAO). In parallel to these bottom-up processes, the top-down control of phytoplankton by copepods decreased over the same time period in the western NASG, following sea surface temperature changes typical of the Atlantic Multi-decadal Oscillation (AMO). While previous studies have hypothesized that climate-driven warming would facilitate seasonal stratification of surface waters and long-term phytoplankton increase in subpolar regions, here we show that deeper mixed layers in the NASG can be warmer and host a higher phytoplankton biomass. These results emphasize that different modes of climate variability regulate bottom-up (NAO control) and top-down (AMO control) forcing on phytoplankton at decadal timescales. As a consequence, different relationships between phytoplankton, zooplankton, and their physical environment appear subject to the disparate temporal scale of the observations (seasonal, interannual, or decadal). The prediction of phytoplankton response to climate change should be built upon what is learnt from observations at the longest timescales.

Can top-down and bottom-up forces explain phytoplankton structure in a subtropical and shallow groundwater-connected lake?

Bottom-up and top-down control of phytoplankton is one of the most important hypothesis that explains and predicts the structure of aquatic community. Our aim was to elucidate whether predation and resource limitation can control phytoplankton composition and abundance in a subtropical shallow lake with groundwater connection to the river system. During 12 months, the lake was sampled at three points. Physico-chemical parameters, phytoplankton and zooplankton were sampled fortnightly, whereas fish were sampled every 3 months. The results showed that Euglenophyta dominated the total biovolume, followed by Dinophyta and Cryptophyta. As for the species composition, Chlorophyta was the dominant group (80 species recorded), followed by phylum Cyanobacteria (26 species recorded). Redundancy analysis (RDA) indicated that temperature and nitrate + nitrite concentration mainly explained biovolume changes, with zooplankton predation not having any measurable effect on phytoplankton during the high-water (HW) period. During low-water (LW) period top-down by fish was more important. At higher taxonomic resolution (species biovolume), phosphorus was another controlling factor. We concluded that phytoplankton in this lake is mainly regulated by hydrological changes as a macrofactor that affects nutrient availability and other environmental conditions. Even though bottom-up top-down forces do not have a central effect, we found evidence of positive nutrient influences at the HW period and fish effect at the LW period.

Using a weight-structured oyster population dynamic model to explore top-down control of coastal water quality in a subtropical embayment

Abstract The natural filtering capacity of oysters and other suspension filter-feeders has seen them put forward as a potential water quality management option. However, the specifics of how many oysters would be required to clean a system are not necessarily straightforward to evaluate because of the size-dependence of oyster physiological rates along with the dynamic coupling that exists between the oysters and the environment. We use a weight-structured shellfish population model and a nutrient–phytoplankton–detritus model to answer the question of how many oysters it would take to clean a large estuary located in Queensland, Australia. Modelling results indicate that improvements in the water quality are not seen until the stocking density of oysters exceeds 0.09 ind. m−3 and that local water quality guidelines are not fully met until the density exceeds 1.80 ind. m−3. At these densities, the corresponding times to filter the entire volume of the study area are 52–38 d (0.09 ind. m−3) and 1.80–1.35 d (1.80 ind. m−3), respectively. This research highlights that regulatory feedback pathways exist between a shellfish population and the water quality constituents that they control through filtration. While the use of oysters and other filter-feeders may be an appealing approach to nutrient management and top-down control of phytoplankton, the practicalities of deploying oysters at a system-scale may be the greatest barrier to this option.

The influence of plant-associated filter feeders on phytoplankton biomass: a mesocosm study

Low phytoplankton biomass usually occurs in the presence of submerged macrophytes, possibly because submerged macrophytes enhance top-down control of phytoplankton by offering a refuge for efficient grazers like Daphnia against fish predation. However, other field studies also suggest that submerged macrophytes suppress phytoplankton in the absence of Daphnia. In order to investigate these mechanisms further, we conducted an outdoor mesocosm experiment to study the effect of submerged macrophytes (Elodea nuttallii) on phytoplankton and zooplankton biomass. The experiment combined four nutrient addition levels (0, 10, 100, and 1000 μg P l−1; N/P ratio: 16) with three macrophyte levels (no macrophytes, artificial macrophytes, and real macrophytes). We inoculated the tanks with species-rich inocula of phytoplankton and zooplankton but excluded fish or macro-invertebrates. Probably due to the lack of predators in the mesocosms, potential grazing rates of pelagic zooplankton (estimated from zooplankton biomass) did not differ between the macrophyte treatment combinations. Compared to the treatment combinations without macrophytes, lower phytoplankton biomass occurred in the treatment combinations with real macrophytes at all the nutrient addition levels and in those with artificial macrophytes at all the nutrient levels except the highest. Significantly, higher abundances of plant-associated filter feeders (Simocephalus vetulus and Ceriodaphnia spp.) occurred in the treatment combinations with real and artificial macrophytes. The estimated potential grazing rate of these plant-associated filter feeders indicated that these filter feeders could be responsible for the lower phytoplankton biomass in the presence of real and artificial macrophytes. Our results suggest that the plant-associated filter feeders may be significant grazers in vegetated shallow lakes.

A comparison of zooplankton densities and biomass in Lakes Peipsi and Võrtsjärv (Estonia): rotifers and crustaceans versus ciliates

The abundance and biomass of ciliates, rotifers, cladocerans and copepods were studied in Lake Peipsi and Lake Vortsjarv, both of which are shallow, turbid and large. Our hypothesis was that in a large shallow eutrophic lake, the ciliates could be the most important zooplankton group. The mean metazooplankton biomass was higher in Peipsi than in Vortsjarv (mean values and SD, 1.8 ± 0.7 and 1.3 ± 0.6 mg WM l−1). In Peipsi, the metazooplankton biomass was dominated by filtrators that feed on large-sized phytoplankton and are characteristic of oligo-mesotrophic waters. In Vortsjarv, the metazooplankton was dominated by species characteristic of eutrophic waters. The planktonic ciliates in both lakes were dominated by oligotrichs. The biomass of ciliates was much greater in Vortsjarv (mean 2.3 ± 1.4 mg WM l−1) than in Peipsi (0.1 ± 0.08 mg WM l−1). Ciliates formed about 60% of the total zooplankton biomass in Vortsjarv but only 6% in Peipsi. Thus, the food chains in the two lakes differ: a grazing food chain in Peipsi and a detrital food-chain in Vortsjarv. Consequently, top-down control of phytoplankton can be assumed to be much more important in Peipsi than in Vortsjarv. When the detrital food chain prevails, the planktonic ciliates become the most important zooplankton group in shallow, eutrophic and large lake. Neglecting protozooplankton can result in serious underestimates of total zooplankton biomass since two-thirds of the zooplankton biomass in Vortsjarv comprises ciliates.

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.

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.

Top-down control of phytoplankton by oysters in Chesapeake Bay, USA: Comment on Pomeroy et al. (2006)

Pomeroy et al. (2006) proposed that temporal and spatial mismatches between eastern oyster filtration and phytoplankton abundance will preclude restored stocks of eastern oysters from reducing the severity of hypoxia in the deep channel of central Chesapeake Bay. We refute this con- tention by presenting arguments, data, and model results, overlooked by these authors. Our analysis indicates that oyster populations living on extensive reefs along the flanks of the mainstem Bay could substantially reduce summer phytoplankton growth and particulate organic carbon deposition to deep waters of the central channel. Because hypoxia in these deep waters is maintained through microbial decomposition of organic carbon generated by summer phytoplankton production, we conclude that reduced carbon fluxes to the deep channel associated with greatly increased oyster grazing could reduce the severity of hypoxia.

Cladocerans versus copepods: the cause of contrasting top–down controls on freshwater and marine phytoplankton

Top-down control of phytoplankton by crustacean mesozooplankton is a cornerstone of freshwater ecology. Apparently, trophic cascades are more frequently reported from freshwater than from marine plankton. We argue that this difference is real and mainly caused by biological differences at the zooplankton-phytoplankton link: cladocerans (particularly Daphnia) in the lakes and copepods in the sea. We derive these conclusions from recent literature and a number of own, similarly designed mesocosm experiments conducted in a lake, a brackish water and a marine site. In all experiments, phytoplankton were exposed to gradients of experimentally manipulated densities of zooplankton, including freshwater copepods and cladocerans, and marine copepods and appendicularians. The suggested reasons for the difference between lake and marine trophic cascades are: (1) Both copepods and cladocerans suppress only part of the phytoplankton size spectrum: cladocerans the small and copepods the large phytoplankton. (2) If not controlled by grazing, small phytoplankton may increase their biomass faster than large phytoplankton. (3) Copepods additionally release small phytoplankton from grazing pressure by intermediate consumers (protozoa) and competitors (predation on appendicularian eggs), while cladocerans do not release large phytoplankton from grazing pressure by any functional group. (4) Cladocerans sequester more of the limiting nutrient than copepods, leaving fewer nutrients available for compensatory growth of ungrazed phytoplankton.

Inhibitory analysis of top-down control: new keys to studying eutrophication, algal blooms, and water self-purification

Top-down control is an important type of interspecies interactions in food webs. It is especially important for aquatic ecosystems. Phytoplankton grazers contribute to the top-down control of phytoplankton populations. The paper is focused on the role of benthic filter feeders in the control of plankton populations as a result of water filtering and the removal of cells of plankton from the water column. New data on the inhibitory effects of surfactants and detergents on benthic filter-feeders (Unio tumidus, U. pictorum, Mytilus galloprovincialis, M. edulis and Crassostrea gigas) are presented and discussed. Importance and efficiency of that approach to the problems of eutrophication and water self-purification is pointed out. Chemical pollution may pose a threat to the natural top-down control of phytoplankton and water self-purification process. The latter is considered an important prerequisite for sustainable use of aquatic resources.

Zooplankton grazing dynamics: top‐down control of phytoplankton and its relationship to an estuarine habitat

AbstractTop‐down control of algal blooms was investigated in the Swan–Canning Estuary, to determine whether the resident zooplankton community was able to substantially reduce phytoplankton biomass through grazing, or whether high phytoplankton biomass effectively inhibited grazing. In situ grazing by zooplankton at two depths in the water column was measured over a diel period on six occasions and examined in relation to the biotic and abiotic habitat conditions.Measured weight‐specific ingestion rates varied with phytoplankton and zooplankton species composition, but were significantly greater at the surface than at 5 m, close to the bottom (ranges of 1·59 to 252·41 and 0·51 to 28·99 µg phytoplankton C µg zooplankton C−1 day−1, respectively). There was no evidence of significant diel changes in ingestion rates of zooplankton. Phytoplankton biomass removed by in situ community grazing ranged between 0·9 and 45·0% of standing stock per hour. The maximum loss occurred when phytoplankton biomass ranged from 249·0 to 320·8 µg phytoplankton C l−1. Zooplankton grazing impact was negatively associated with high phytoplankton and low dissolved oxygen in bottom waters. The implications of, and mechanisms associated with, top‐down control of algal biomass in this estuary are discussed in relation to plankton succession and the physical environment. Copyright © 2001 John Wiley & Sons, Ltd.

Advances in Ecological Studies on Zooplankton

Advances in ecological stuides on zooplankton were reviewed based on the published papers and authors' studies. Contents include long-term ecology, dynamics changes of zooplankton, predation ecology and effects of environmental factors such as temperature, food resources on zooplankton. Long-term studies on zooplankton are very important to understand changes of ecosystem, especially those driven by slow processes, rare or episodic events and highly variable, stable or complex processes. The major objectives of long-term investigations are to provide reliable estimates of base-line variation, to detect long-term trends in the mean level of base-line, to detect rare events and to provide information for meaningful, testable hypotheses. A lot of research results demonstrated that changes in zooplankton community structure were determined largely by fish in the water bodies, with high stocking rate of planktivorous fish, and also by predation of invertebrate animals such as some copepods. Water temperature, food resources etc. may cause a stable zooplankton community. Among the freshwater zooplankton community, Daphnia and Cyclops vicinus may be the key groups. Because Daphnia plays a key role in the top-down control of phytoplankton, and C. vicinus are key factors in aquatic food chain during the period of their maximum abundance. In addition, several research topics such as molecule biology of zooplankton, zooplankton ecology of the surface microlayer, influence of engineering water conservancy project on ecosystem and zooplankton, and batch culture of zooplankton were discusses.

A review of the feedbacks between bivalve grazing and ecosystem processes

This paper gives an overview of interactions betweenbivalve grazing and ecosystem processes, that mayaffect the carrying capacity of ecosystems for bivalvesuspension feeders. These interactions consist of anumber of positive and negative feedbacks.Bivalve grazing can result in local food depletion,which may negatively influence bivalve growth. On alarger scale, it may induce a top-down control ofphytoplankton biomasss, and structural shifts inphytoplankton composition. In the case of harmfulalgal blooms, phytoplankton may negatively affectbivalve grazing rates.The processing of large amounts of particulate mattermay change nutrient cycling on the scale of estuaries,and can result in changes in the inorganic nutrientpool available for phytoplankton, through regenerationand reduced storage of nutrients in algal biomass.This can reduce nutrient limitation of thephytoplankton and stimulate algal growth rates.Observations from mesocosm studies suggest that apositive feedback from bivalve grazing onphytoplankton growth may also change the physiologicalstate of the algae and improve food quality.