by Brian Tomasik
First published: 2 Jan. 2017; last update: 2 Jan. 2017
The net impact of eutrophication on aggregate invertebrate populations in freshwater lakes and rivers is unclear, but evidence points toward greater nutrient inputs causing greater total metazoan zooplankton populations (but reduced mayfly populations). I wonder if these trends depend a lot on whether and how anoxic the lake is.
Note: This page is a work in progress and isn't yet comprehensive.
In many cases, human impacts on the environment reduce the net primary productivity (NPP) of affected regions. In general, this can be expected to lower total animal populations as well, since animals feed (directly and indirectly) on plant biomass.
Eutrophication is a rare situation where human activity substantially increases NPP. Thus, a priori, we would expect eutrophication to increase animal populations. However, in lakes that become sufficiently anoxic due to decomposition of organic matter, perhaps this trend would reverse, given that animals (unlike some microbes) require oxygen to eat organic matter.
Chlorophyll sometimes correlates with zooplankton
Orsi and Mecum (1986)
Orsi and Mecum (1986) examined zooplankton in the Sacramento-San Joaquin delta. They found (p. 337):
The decline in overall zooplankton abundance from 1972 to 1978 was roughly paralleled by a decrease in the mean chlorophyll a concentration from 18.5 μg l-1 in 1972 to 4.5 μg l-1 in 1978. Simple linear correlation coefficients between zooplankton density and chlorophyll a concentrations for 1972-1978 were 0.864 (p < 0.05) for rotifers, 0.522 (not significant) for cladocerans, and 0.941 (p < 0.01) for copepods (Figs. 15, 16, and 17).
Pace et al. (1992)
Pace et al. (1992) examined "zooplankton in the tidal freshwater portion of the Hudson River" in New York, USA (p. 1060). The authors found a correlation of Bosmina with chlorophyll, though not a correlation of cyclopoid copepods with chlorophyll (pp. 1064, 1066):
We tested whether spatial variation was related to food resources by comparing average chlorophyll concentrations with averages of zooplankton abundance across all surveys. Using average abundance values for each station removes any effect of seasonally important factors such as temperature and discharge. Patterns of spatial variation for Bosmina were strongly related to differences among stations in chlorophyll if the most seaward station (Haverstraw Bay) was not considered (Fig. 6). In the case of Haverstraw, the abundance of Bosmina was low relative to the chlorophyll concentration. This section of the river is probably near the seaward limit of Bosmina, a freshwater species. Densities of cyclopoid copepods were not significantly related to differences in chlorophyll among stations.
The authors also mention (p. 1066) that "It has been previously documented for lakes that zooplankton biomass varies positively as a function of algal biomass (McCauley and Kalff 1981)."
Trophic status and zooplankton
This study says in its "Introduction" that "Eutrophication caused by water pollution results in excessive propagation of zooplankton cyclops in waterbodies" (p. 1826).
Pinto-Coelho et al. (2005)
Pinto-Coelho et al. (2005) explain in their "Introduction" (p. 349):
Zooplankton have long been used as an indicator of lake trophy (Gannon and Stemberger 1978; Bays and Crisman 1982; Pejler 1983). Nonetheless, variable responses of zooplankton to trophic status are common (Ravera 1996), perhaps in part because zooplankton also respond to other environmental factors, such as lake water chemistry, shoreline disturbances, and watershed land use (Pinel-Alloul et al. 1990; Stemberger and Lazorchak 1994; Patoine et al. 2000) as well as levels of vertebrate and invertebrate predation (Hulot et al. 2000).
Pinto-Coelho et al. (2005) summarize more previous literature in the "Discussion" (p. 359):
High densities of total zooplankton (over 103 individuals·L–1) are common in tropical waters. Crustacean zooplankton, especially small cladocerans and cyclopoids, can often reach such densities in hypereutrophic lakes, reservoirs, and fertilized ponds of hatchery stations (Branco and Senna 1996). Even the small calanoid Scolodiaptomus corderoi can occur and even form large populations in the most eutrophic reservoirs of Brazil (Reid et al. 1988).
Pinto-Coelho et al. (2005) then present their findings (p. 359):
Our study confirmed that more nutrient-enriched lakes or reservoirs support greater crustacean zooplankton density and biomass. This is valid for both temperate and tropical regions. The regression models relating biomass of total crustacean zooplankton to [total phosphorus] TP were significant for most regions (excepting only Florida lakes) with R2 varying between 0.45 and 0.77. The predictive power of our large-scale regression models is similar to that of other studies (R2 = 0.63–0.86) done either at large geographic scales including 44 lakes mostly from temperate regions of North America and Europe (Hanson and Peters 1984) or at small regional scales in 16 nutrient-poor lakes in southcentral Ontario (Yan 1986) and 12 mesoeutrophic lakes in southern Quebec (Pace 1986). Patalas (1972) also found a significant regression (R2 = 0.92) between crustacean abundance and phosphorus loading in the Great Lakes.
Eutrophication kills mayflies
A famous study involving mayflies was conducted in Lake Erie during the 1950s and early 1960s. Lake Erie was badly polluted at that time, especially by organic debris associated with sewage and algal growths, the decomposition of which consumed most of the oxygen in the waters of deeper parts of the lake. The development of anoxic conditions resulted in mass die-offs of nymphs of the mayflies Hexagenia rigida and H. limbata, which were previously extremely abundant.
If eutrophication prevents trillions of mayflies from being born, this seems good.
Degree of anoxia
My intuition would be that total invertebrate animal populations should increase with eutrophication up to a point at which lake oxygen levels become sufficiently low, after which total animal populations would (sharply) decline. The quotes presented above haven't distinguished whether the lakes being described were oxic or anoxic. It would be helpful to look into this distinction in further research.
This source discusses gastrotrichs, which are small aquatic animals. It reports: "Kisielewski[54,55] showed that gastrotrich density and species richness are positively correlated with the productivity of the habitat" (p. 165). Moreover: "Gastrotrichs are among the few animals commonly found in anaerobic environments[17,66,98], remaining abundant even during extended periods (months) of anoxia" (p. 165). Are gastrotrichs more numerous in even anoxic eutrophic lakes than in oxic oligotrophic lakes??