Abstract
Zooplankton are relatively small in size in the subtropical regions. This characteristic has been attributed to intense predation pressure, high nutrient loading and cyanobacterial biomass. To provide further information on the effect of predation and cyanobacteria on zooplankton size structure, we analyzed data from 96 shallow aquaculture lakes along the Yangtze River. Contrary to former studies, both principal components analysis and multiple regression analysis showed that the mean zooplankton size was positively related to fish yield. The studied lakes were grouped into three types, namely, natural fishing lakes with low nutrient loading (Type1), planktivorous fish-dominated lakes (Type 2), and eutrophic lakes with high cyanobacterial biomass (Type 3). A marked difference in zooplankton size structure was found among these groups. The greatest mean zooplankton size was observed in Type 2 lakes, but zooplankton density was the lowest. Zooplankton abundance was highest in Type 3 lakes and increased with increasing cyanobacterial biomass. Zooplankton mean size was negatively correlated with cyanobacterial biomass. No obvious trends were found in Type 1 lakes. These results were reflected by the normalized biomass size spectrum, which showed a unimodal shape with a peak at medium sizes in Type 2 lakes and a peak at small sizes in Type 3 lakes. These results indicated a relative increase in medium-sized and small-sized species in Types 2 and 3 lakes, respectively. Our results suggested that fish predation might have a negative effect on zooplankton abundance but a positive effect on zooplankton size structure. High cyanobacterial biomass most likely caused a decline in the zooplankton size and encouraged the proliferation of small zooplankton. We suggest that both planktivorous fish and cyanobacteria have substantial effects on the shaping of zooplankton community, particularly in the lakes in the eastern plain along the Yangtze River where aquaculture is widespread and nutrient loading is high.
Highlights
The structure of shallow lake ecosystems could be influenced by human activities such as aquaculture and nutrient enrichment [1]
Zooplankton community structure A total of 35 zooplankton taxa were recorded in all lakes, but the dominant species varied in the three types of lakes
Fish yield was strongly positively correlated with the axis of PCA1 axis, whereas the Secchi depth (SD) and total nitrogen (TN): total phosphorus (TP) ratio were negatively correlated with this axis
Summary
The structure of shallow lake ecosystems could be influenced by human activities such as aquaculture and nutrient enrichment [1]. Predation by fish is thought to induce shifts in the size structure and species composition of zooplankton [4,5]. In subtropical and tropical shallow lakes, zooplankton communities often comprise small cladocerans, copepods and rotifers [6,7,8]. These lakes are always found to be dominated by abundant omnivorousplanktivorous fish. A low biomass and small mean size of zooplankton are commonly observed in such lakes [9,10]. Experimental studies have documented that a high level of fish predation is responsible for the observed structural patterns of the zooplankton communities in subtropical areas [11,12]. The absence of large and more efficient filter-feeding zooplankton (e.g., Daphnia) results in weak phytoplankton control and low water clarity [10,15]
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