Top-down and bottom-up effects on zooplankton size distribution in a deep stratified lake

Abstract

Trophic interactions in the pelagic area of lakes and the opposing effects of fish feeding (top-down) and phytoplankton biomass (bottom-up) on zooplankton communities are central topics in limnology. We hypothesized that zooplankton size distributions should be a more sensitive approach to disentangle top-down and bottom-up effects than the commonly measured zooplankton biomass. We examined zooplankton size distributions from 148 samples collected during summer months in the upper and lower pelagic layers of a deep mesotrophic lake among 13 years of sampling. Top-down effects, namely fish size and biomass, and bottom-up effects, including water temperature and total phosphorus and chlorophyll a concentrations, were considered. To add robustness to our analyses, we expressed the zooplankton size distributions as size spectra based on log-binning, as continuous size spectra and by the size diversity, a measure that has been developed to mimic taxonomic diversity indices. Among numerous regressions tested, significant top-down or bottom-up effects could rarely been detected. Our results indicate that the overall zooplankton size distribution was not significantly affected by fish predation and lake productivity measured as total phosphorus or chlorophyll a concentration. However, we found negative correlations between fish biomass and the preferred zooplankton prey, including Bosmina longirostris, Daphnia cucullata and nauplii in the upper depth layer. However, due to their small body size, low biomass and therefore relative small contribution to the zooplankton size distribution, predation on preferred zooplankton species did not translate into a statistically significant modification of the entire size spectrum. Consequently, the size spectrum seems to be relatively robust against predation effects, but might reflect the lake-wide energy availability and transfer efficiency in the food web.

Size Spectra of Pelagic Fish Populations in a Deep Lake—Methodological Comparison between Hydroacoustics and Midwater Trawling

“…This trend was not obvious in exponent b from the trawl catches, but we indeed recorded a bigger number of larger coregonids (>30 g) in trawl catches in recent years and found single individuals with more than 50 g not recorded in any of the previous years since the start of monitoring in 2005. These changes in size distribution may reflect an enhanced growth rate of coregonids, likely caused by the recently accelerated eutrophication of Lake Stechlin [29]. Furthermore, [29] revealed that hydroacoustically determined fish abundances were negatively correlated with zooplankton taxa forming the diet of the coregonids, whereas no such correlations were found for fish biomasses as obtained by trawling.…”

“…These changes in size distribution may reflect an enhanced growth rate of coregonids, likely caused by the recently accelerated eutrophication of Lake Stechlin [29]. Furthermore, [29] revealed that hydroacoustically determined fish abundances were negatively correlated with zooplankton taxa forming the diet of the coregonids, whereas no such correlations were found for fish biomasses as obtained by trawling. This could indicate that hydroacoustics may reflect the actual state, including abundance, biomass, and size distribution, of the fish community better than trawling, allowing for more reliable insights into ecological interactions.…”

“…Furthermore, considering previous studies on the reasonably good correspondence of fish biomass estimates between net fishing and hydroacoustic [26,27], and between echo integration and tracking [24,28], we assumed that SS generated by all methods would also be reasonably well correlated. Finally, because Lake Stechlin has shown signs of accelerated eutrophication in recent years [29,30], we expected that the size distribution would become shallower, reflecting faster growth rates of coregonids in response to higher primary production (compare [31]).…”

Genetic relationships between sympatric and allopatric Coregonus ciscoes in North and Central Europe

“…The genetic differentiation between the autumn-and spring-spawners in both lakes was weak according to earlier studies [34]. Furthermore, both lakes underwent strong eutrophication during the last decades [36][37][38]. Therefore, the putatively endemic spring-spawners in both lakes may be exposed to a high risk of extinction.…”

Impact of river-reservoir hybrid system on zooplankton community and river connectivity

“…The transformed littoral zone, dredged sediment, and increased water volume can change habitat characteristics (Ko et al, 2020). Various factors need to be considered to understand the changes in zooplankton communities due to RRHS (Kim et al, 2012;Braun et al, 2021), but understanding all these factors can be challenging. Therefore, considered only two factors: rst, the size of plankton, which simultaneously re ects the bottom-up effect leading to water quality-phytoplankton-zooplankton and the top-down effect leading to sh-zooplankton.…”