Chlorophyll-nutrient relationship changes with lake type, season and small-bodied zooplankton in a set of subtropical shallow lakes

Abstract

Phytoplankton biomass is generally measured as chlorophyll-a concentration (Chla), and is well known to be sensitive to nutrients and large zooplankton. However, whether nutrient was still be a better predictor of Chla when potential affecting variables were taken together, and whether the critical TN:TP (total nitrogen: total phosphorus, by mass) threshold for classifying nutrient limitation varied with lake type remain uncertain. Furthermore, it remains largely unknown to what extent small zooplankton (SZ) can affect the nutrients-Chla relationships. Here, an investigation on how Chla responded to abiotic factors and SZ dynamics was performed in 21 subtropical shallow lakes along the lower reach of Chinese Yangtze River. These lakes were subdivided into two lake types including urban lakes featured by a low water exchange rate, and suburban lakes with hydrological connectivity. Chla and TP:Chla ratio were generally higher and lower in urban lakes than those in suburban lakes, respectively. Various analyses revealed that TP and water temperature were the key factors regulating Chla in urban and suburban lakes, respectively. Across urban lakes, TP accounted for >50% of the Chla variances in spring, autumn, and winter, but only explained 28% of summer Chla. However, if the sampling sites where SZ density was >17.5 times higher than that in spring were removed, the determination coefficient (R2) in the linear regression analysis between TP and summer Chla increased (from 0.28 to 0.57), and the Akaike Information Criterion decreased. Furthermore, TP:Chla was positively correlated with SZ in urban lakes in spring when the highest density of SZ (HSZ): the lowest density of SZ (LSZ) ratio was 456 (>17.5), while that interrelationship was not found in summer with the HSZ:LSZ ratio of 17.5. Therefore, the weak TP-summer Chla relationship in urban lakes was primarily caused by SZ when their density exceeded a 17.5-fold increase, i.e. small zooplankton could adopt “quantity-driven” strategy for phytoplankton control. For further strengthening the eutrophication management and remediation in subtropical urban lakes, it is critical to recover aquatic vegetation that can not only absorb P directly but provide refuge for zooplankton instead of dredging the sediments completely. In addition, we recommended that TN:TP ratio exceeding 20.0 could be the threshold identifying P-limitation in urban lakes rather than suburban lakes, and transparency can be used as rough indicators for measuring TP and Chla concentrations (<25 μm) in urban lakes.