Feeding resources and key nutrients of commercial fish in the Ural-Caspian Basin

Global changes are causing decreases in inorganic nitrogen (N) concentrations, increases in coloured dissolved organic carbon (DOC) concentrations, and decreases in dissolved inorganic N to total phosphorus ratios (DIN:TP) in northern lakes. The effects of these changes on phytoplankton and zooplankton biomass and the N:P recycling ratio of zooplankton remain unresolved. In 33 Swedish headwater lakes across subarctic‐to‐boreal gradients with different levels of N deposition (low N in the north [Västerbotten, boreal; Abisko, subarctic] vs. high N in the south [Värmland, boreal; Jämtland, subarctic]), we measured water chemistry, phytoplankton biomass (chlorophyll‐a [Chl‐a], Chl‐a:TP), seston mineral quality (C:P, N:P), as well as zooplankton biomass, community composition, and C:N:P stoichiometry. We estimated nutrient imbalances and the N:P recycling ratios of zooplankton using ecological stoichiometry models. There was a large‐scale gradient from low lake DIN and DIN:TP in the north to high DIN and DIN:TP in the south, with lower DIN:TP in lakes coinciding with higher DOC within each region. Lower lake DIN was associated with lower phytoplankton biomass (lower Chl‐a:TP). Lower lake DIN:TP was associated with richer seston mineral quality (lower seston C:P and N:P) and higher zooplankton biomass. Zooplankton community composition differed in the north vs. south, with a dominance of N‐requiring calanoid copepods with high N:P in the north and P‐requiring cladocerans with low N:P in the south. Also, greater differences in zooplankton community composition were found between subarctic regions (with lower DOC) than between boreal regions (with higher DOC), suggesting that increases in lake DOC and associated declines in lake DIN:TP reduce differences in zooplankton community composition. The combination of lower lake DIN, higher lake DOC, and lower lake DIN:TP led to reduced zooplankton N:P recycling ratios, possibly by reducing seston N:P and/or by enhancing calanoid copepod dominance in the zooplankton community. Our findings suggest that the combination of declining N deposition and increasing lake browning in northern high‐latitude lakes will reduce phytoplankton biomass, but will concurrently enhance seston mineral quality and probably also zooplankton biomass and their recycling efficiency of P relative to N.

Temporal variation in zooplankton and phytoplankton community species composition and the affecting factors in Lake Taihu—a large freshwater lake in China

Lake Balaton, the largest lake in Central Europe, underwent severe eutrophication from the 1960s to the 1990s, due to phosphorus loadings from external anthropogenic sources. The subsequent and complex eutrophication control and lake restoration program resulted in a significant decrease in the external phosphorus loading to the lake. Consequently, Lake Balaton has been returning to its former meso-eutrophic character. In this paper, we explore the long-term dynamics of chlorophyll a (Chl a) concentration, a proxy for phytoplankton biomass, and zooplankton biomass in Lake Balaton during its re-oligotrophication period from 2001 to 2017, and attempt to draw some conclusions on the subsequent changes in the fish stock. We found a proportional decrease in zooplankton and phytoplankton biomasses at moderate phytoplankton levels. However, below a certain phytoplankton concentration (< 10 μg l−1 Chl a), the decrease in phytoplankton biomass was not coupled with a further decline in zooplankton biomass because the fraction of small phytoplankton, edible for zooplankton, showed a much smaller decrease in biomass compared with large non-edible phytoplankton. Thus, improvements in water quality (i.e., reduced nutrient loading), partly via concomitant changes in the phytoplankton size distribution, did not cause a large difference in the fish stock in this shallow lake.

The relationship between zooplankton biomass and phytoplankton biomass can provide insight into the structure and function of lake biological communities. We used a Bayesian network model to analyze a continental-scale data dataset to estimate changes in the relationship between zooplankton (Z) and phytoplankton (P) biomasses along a eutrophication gradient. The Bayesian network model allowed us to combine two different measurements of phytoplankton biomass (chlorophyll a concentration and directly observed biovolume) to improve the precision of estimates of true biomass within each sample. The model also allowed us to estimate separate relationships between P and zooplankton abundance and between P and mean individual zooplankton biomass and then to combine these two relationships into an estimate of seasonal mean zooplankton biomass. The resulting analysis indicated that seasonal mean zooplankton biomass increased proportionally with phytoplankton biomass in oligotrophic lakes, yielding a constant ratio between Z and P and suggested that bottom-up forces determined zooplankton biomass in these systems. In eutrophic lakes, seasonal mean zooplankton biomass was nearly constant with increases in phytoplankton biomass, yielding a decrease in the ratio between Z and P with increasing eutrophication. Bottom-up forces, as quantified by an increase in the proportion of cyanobacteria, accounted for approximately one fifth of the residual variance in the model as the relationship between Z and P changed from direct proportionality in oligotrophic lakes to the nearly constant value of Z observed in eutrophic lakes, suggesting that a combination of both top-down and bottom-up forces likely determined zooplankton biomass in eutrophic lakes.

Based on the data of zooplankton, phytoplankton, and water environmental factors in different seasons in a typical tributary reservoir in the Three Gorges Reservoir Region (Changshou Lake), this study explored the relationships between the structures of the zooplankton and phytoplankton communities and environmental factors using Pearson correlation analysis. The results showed that there were a total of 107 species of 8 phyla of phytoplankton, and cyanobacteria was the most critical constituent with a relative abundance of 61%. The dominant populations included Phormidium tenue, Merismopedia punctata, and Anabaena oscillarioides. A total of 82 species of 4 phyla of zooplankton were identified, and rotifers was the most abundant with a relative abundance of 88%. The dominant populations included six species, such as Keratella cochlearis, Asplanchna priodonta, and Asplanchna girodi. The spatial differences in the abundances, biomass, and biodiversity indexes of zooplankton and phytoplankton were not significant, whereas the seasonal differences in all the other indexes were significant, except for the zooplankton biodiversity indexes. The abundance of phytoplankton was the highest in summer, followed by spring, and it was the lowest in winter. The maximum abundance of zooplankton occurred in spring, and the biomass of zooplankton and phytoplankton in spring was significantly higher than that in winter. The number of phytoplankton species and the Shannon-Wiener index, Pielou's uniformity index, and Margalef richness index in summer were significantly lower than those in winter and spring. The water quality evaluation showed that Changshou Lake was in a clean to oligo-pollution state in winter and spring and a moderate-pollution state in summer, thereby suggesting that Changshou Lake was in an overall eutrophic state. The environmental factors, including Chla, DOC, nutrients (TP, NO2--N, NO3--N, and NH4+-N), DO, Eh, and T, influenced the structures of the zooplankton and phytoplankton communities in Changshou Lake, and there were seasonal differences in the environmental factors.

We investigated the seasonal influence of an effective size-selective planktivore, the anadromous alewife (Alosa pseudoharengus Wilson), on summer zooplankton and phytoplankton communities in a series of lakes in Maine, USA (4 with and 4 without alewife) that ranged from oligotrophic to eutrophic to determine the role of lake trophic state in influencing the relative strength of top–down forces on phytoplankton biomass. Predation by young of the year alewife reduced the mean body length of cladoceran and copepod biomass from spring to summer in alewife lakes, while mean body length remained unchanged in nonalewife lakes. Cladoceran biomass decreased substantially from spring to summer in 3 of 4 alewife lakes, increasing only in the most eutrophic lake. Conversely, cladoceran biomass increased from spring to summer in 3 of 4 nonalewife lakes. Predation by alewife on zooplankton did not have consistent cascading effects on phytoplankton biomass. Analysis of planktonic biomass ratios (phytoplankton biomass: zooplankton biomass) suggests that cascading effects were stronger in oligotrophic systems and weakened with increasing trophic status, as ratios in alewife and nonalewife lakes converged at higher total phosphorous levels. Our results suggest that lake trophic status may influence the relative importance of top–down control of both zooplankton and phytoplankton biomass. The mechanisms that explain this pattern remain elusive and likely require additional efforts to estimate alewife densities, rates of zooplankton and phytoplankton production relative to consumption, and the influence of other physical lake features.

Many continental saline lakes are under the effects of salinity increase and anthropogenic eutrophication exacerbated by global change. The response of the food web to these drivers of change is not straightforward. To understand the consequences of salinity and eutrophication interactive effects on the food web, we studied the seasonal dynamics of zooplankton and phytoplankton and water quality parameters in 20 lakes of different salinity (from freshwater to hypersaline) and nutrient status (from oligotrophic to eutrophic) located in southern Siberia. We observed a pronounced bottom-up effect of nutrients, which induced an increase in the biomass of phytoplankton and zooplankton and a decline in water quality. A significant decrease in the species abundance of zooplankton was observed at a threshold salinity of 3 g L−1 and the disappearance of fish at 10 g L−1. The top-down effect induced by salinity manifested itself in an increase in the biomass of zooplankton with the disappearance of fish, and in the change of the size distribution of phytoplankton particles with an increase in the proportion of cladocerans in the zooplankton. Even though we observed that with the salinity increase the food web in saline lakes transformed from three-trophic to two-trophic without fish, we conclude that in the salinity range from 10 to 20–30 g L−1 this transition in most cases will not increase the ability of zooplankton to control phytoplankton. Interactive effects of salinity and eutrophication strongly depend on the size and depth of the lake, as deep stratified lakes tend to have a better water quality with lower biomasses of both phyto- and zooplankton. Thus, the salinity per se is not the driver of the decline in water clarity or the uncontrolled development of phytoplankton. Moreover, for deep lakes, salinity may be a factor affecting the stability of stratification, which mitigates the consequences of eutrophication. Thus, small shallow lakes will be the most vulnerable to the joint effect of salinity increase and eutrophication with the degradation of ecosystem functioning and water quality at moderate salinities of 3–20 g L−1.

Contrasting currents drive geographic variability in the biomass of Pacific saury (Cololabis saira), zooplankton, and phytoplankton in the northwestern Pacific

Seasonal Changes in Zooplankton Biomass and Grazing in a Temperate Estuary, South Africa

Eutrophication and rising water temperature in freshwaters may increase the total production of a lake while simultaneously reducing the nutritional quality of food web components. We evaluated how cyanobacteria blooms, driven by agricultural eutrophication (in eutrophic Lake Köyliöjärvi) or global warming (in mesotrophic Lake Pyhäjärvi), influence the biomass and structure of phytoplankton, zooplankton, and fish communities. In terms of the nutritional value of food web components, we evaluated changes in the ω‐3 and ω‐6 polyunsaturated fatty acids (PUFA) of phytoplankton and consumers at different trophic levels. Meanwhile, the lakes did not differ in their biomasses of phytoplankton, zooplankton, and fish communities, lake trophic status greatly influenced the community structures. The eutrophic lake, with agricultural eutrophication, had cyanobacteria bloom throughout the summer months whereas cyanobacteria were abundant only occasionally in the mesotrophic lake, mainly in early summer. Phytoplankton community differences at genus level resulted in higher arachidonic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) content of seston in the mesotrophic than in the eutrophic lake. This was also reflected in the EPA and DHA content of herbivorous zooplankton (Daphnia and Bosmina) despite more efficient trophic retention of these biomolecules in a eutrophic lake than in the mesotrophic lake zooplankton. Planktivorous juvenile fish (perch and roach) in a eutrophic lake overcame the lower availability of DHA in their prey by more efficient trophic retention and biosynthesis from the precursors. However, the most efficient trophic retention of DHA was found with benthivorous perch which prey contained only a low amount of DHA. Long‐term cyanobacterial blooming decreased the nutritional quality of piscivorous perch; however, the difference was much less than previously anticipated. Our result shows that long‐term cyanobacteria blooming impacts the structure of plankton and fish communities and lowers the nutritional quality of seston and zooplankton, which, however, is mitigated at upper trophic levels.

We aimed to investigate the influence of environmental factors and predict zooplankton biomass and abundance in shallow eutrophic lakes. We employed time series of zooplankton and environmental parameters that were measured monthly during 38 years in a large, shallow eutrophic lake in Estonia to build estimates of zooplankton community metrics (cladocerans, copepods, rotifers, ciliates). The analysis of historical time series revealed that air temperature was by far the most important variable for explaining zooplankton biomass and abundance, followed, in decreasing order of importance, by pH, phytoplankton biomass and nitrate concentration. Models constructed with the best predicting variables explained up to 71% of zooplankton biomass variance. Most of the predictive variables had opposing or antagonistic interactions, often mitigating the effect of temperature. In the second part of the study, three future climate scenarios were developed following different Intergovernmental Panel on Climate Change (IPCC) temperature projections and entered into an empirical model. Simulation results showed that only a scenario in which air temperature stabilizes would curb total metazooplankton biomass and abundance. In other scenarios, metazooplankton biomass and abundance would likely exceed historical ranges whereas ciliates would not expand. Within the metazooplankton community, copepods would increase in biomass and abundance, whereas cladocerans would lose in biomass but not in abundance. These changes in the zooplankton community will have important consequences for lake trophic structure and ecosystem functioning.

Despite the rapidly changing winter conditions in temperate ecosystems, little attention has been devoted to the effects of these changes on lake ecology. Few studies on the seasonal changes in abundance and biomass of the major groups of the metazooplankton community (i.e. rotifers, cladocerans and copepods) in northern oligotrophic lakes include data from the ice-covered winter months. This study reports monthly variation in zooplankton abundance and biomass from June 2010 to October 2011, including winter, in an oligotrophic, subalpine lake in southeastern Norway (Lake Atnsjøen). Changes in rotifer, cladoceran, copepod, and total zooplankton abundances and biomass were related to seasonal variation in water temperature and phytoplankton biomass by means of ordination analysis. The zooplankton abundance and biomass were much lower in winter than during the open water season. However, an under-ice phytoplankton bloom occurred during the final winter months, when snow cover and ice thickness were reduced and (presumably) light penetration increased, leading to an increase in abundance of copepod nauplii. Winter zooplankton abundance was dominated by copepods and rotifers, while winter zooplankton biomass was dominated by copepods and cladocerans. Both phytoplankton and zooplankton had two biomass peaks in 2010 and one peak in 2011. Rotifers dominated zooplankton abundance with a peak in August and total zooplankton abundance followed a similar pattern. In contrast, cladocerans dominated zooplankton biomass with a peak in July and total zooplankton biomass also peaked at this time. Rotifer and total zooplankton abundance and rotifer biomass were most closely correlated to water temperature. However, cladoceran biomass and total biomass were most closely correlated with phytoplankton biomass, but also appeared to be dependent on other carbon sources. Estimates of non-phytoplankton particulate organic carbon indicated that this part of the carbon pool could be an additional food source for zooplankton particularly in early and mid-winter. The longer growing season in 2011 than in 2010, owing to earlier ice-off in 2011, may have contributed to higher phytoplankton and zooplankton biomass in 2011. With climate warming, this is an expected change in temperate lake ecosystems.

Zooplankton biomass and species composition were monitored in Cat Arm Lake and Reservoir on Newfoundland's Great Northern Peninsula from 1983 to 1993. Zooplankton biomass increased approximately 19-fold in the oligotrophic reservoir following impoundment in 1984, relative to biomass in the preexisting lake. Microcrustaceans Cyclops scutifer and Daphnia dubia, either rare or absent from Cat Arm Lake prior to impoundment, were consistently measurable components of the zooplankton community by 1993. Similar changes elsewhere have been attributed to both increased water retention time and enhanced phytoplankton biomass, factors whose effects are usually interdependent. In Cat Arm, there were no increases in either phytoplankton biomass or primary productivity during the first 3 years of impoundment, and natality of the dominant cladoceran, Daphnia catawba, did not increase. Summer water retention time increased from preimpoundment levels of 4 days in 1983 to 338 days in 1993. Zooplankton biomass was significantly correlated with water retention time (Spearman's rs = 0.72, p = 0.04) and showed no significant correlation with phytoplankton biomass, primary productivity, nutrient concentrations, pH, colour, or epilimnetic temperature. Changes in the zooplankton community in this subarctic system can thus be attributed most directly to a decrease in losses due to washout.

Eutrophication, i.e. increasing level of nutrients and primary production, is a central environmental change of lakes globally with wide effects on food webs. However, how eutrophication affects the synthesis of physiologically essential biomolecules (omega-3 fatty acids) and their transfer to higher trophic levels at the whole food web level is not well understood. We assessed food web (phytoplankton, zooplankton, and fish) biomass, community structure and fatty acid content (eicosapentaenoic acid [EPA], and docosahexaenoic acid [DHA]), together with fatty acid specific primary production in 12 Finnish boreal lakes covering the total nutrient gradient from oligotrophic to highly eutrophic lakes (4–140 μg TP l−1; 413–1814 μg TN l−1). Production was measured as the incorporation of 13C-NaHCO3 into phytoplankton fatty acids and differentiated into volumetric production (production per litre of water) and productivity (production per phytoplankton biomass). Increases in nutrients led to higher biomass of phytoplankton, zooplankton and fish communities while also affecting community composition. Eutrophication negatively influenced the contribution of phytoplankton biomass preferentially grazed by zooplankton (<35 μm). Total volumetric production saturated at high phytoplankton biomass while EPA volumetric production presented a logarithmic relationship with nutrient increase. Meanwhile, total and EPA productivity had unimodal responses to this change in nutrients. DHA volumetric production and productivity presented large variation with increases in total phosphorus, but a unimodal model best described DHA changes with eutrophication. Results showed that eutrophication impaired the transfer of EPA and DHA into zooplankton and fish, showing a clear negative impact in some species (e.g. perch) while having no effect in other species (e.g. roach, ruffe). Results show non-linear trends in fatty acid production and productivity peaking at nutrient concentrations 22–35 μg l−1 TP followed by a gradual decrease.

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 269:173-183 (2004) - doi:10.3354/meps269173 Is the comb jelly really to blame for it all? Mnemiopsis leidyi and the ecological concerns about the Caspian Sea Martin Bilio1,*, Ulrich Niermann2 1Am Hirschsprung 10, 61462 Königstein, Germany 2Am Sackenkamp 37, 23374 Heiligenhafen, Germany *Email: martin.bilio@t-online.de ABSTRACT: The appearance in 1995 of the alien ctenophore Mnemiopsis leidyi in the Caspian Sea attracted considerable attention. In 1989, enormous mass development of the comb jelly in the Black Sea coincided with a breakdown of the commercially highly important anchovy Engraulis encrasicholus fishery in this area. This collapse probably resulted from multiple factors, among which overfishing and food competition from the ctenophore played a prominent role, enhanced by predation of M. leidyi on the early life stages of the anchovy. The abundance of food due to the depletion of the anchovy stock permitted a population explosion of M. leidyi. Subsequently, reduced fishing pressure allowed recovery of the anchovy stock and led to a new peak in anchovy landings in the Black Sea in 1995. Moreover, changes in the atmospheric and oceanic patterns in the northern hemisphere in the second half of the 1980s could have altered the composition of the phyto- and zooplankton communities and thus the food base of the small pelagic fish species. In the second half of the 1990s, the invasion of the Black Sea by another alien ctenophore, Beroe ovata, preying on ctenophores, raised hopes that this species could control M. leidyi. However, it is uncertain whether the preying capacity of B. ovata is sufficient to control M. leidyi in other than locally favourable conditions. At the present time, judging by the recent development in the Black Sea, the invasion of the Caspian Sea by M. leidyi does not yet seem to have reached maximum intensity. More consistent and comparable investigations in the Caspian and Black Seas are necessary to improve the protection, management and exploitation of the stocks of small pelagic fishes, in order to increase their capacity to resist and survive unforeseeable disturbances of their ecosystem by invaders such as M. leidyi. In view of the new invasion by this ctenophore, research into the exploitation of the Caspian kilka Clupeonella spp. stocks and the influence of atmospheric teleconnection patterns on ecological conditions in the Caspian Sea is of particular importance. KEY WORDS: Bioinvasion · Caspian Sea · Black Sea · Ctenophores · Mnemiopsis leidyi · Beroe ovata · Anchovy fishery · Kilka fishery · Regime shift Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 269. Online publication date: March 25, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.