Temporal variations of chlorophyll-a concentration and their relationships with abiotic and biotic factors in two small shallow lakes

Abstract

Identifying the factors affecting the variations of chlorophyll-a concentration is very important in assessment and management of water quality in lakes. Most of previous studies focused on the relationships between chlorophyll-a concentration and abiotic factors, while paid little attention on the effects of biotic ones. To examine the relative importance of the abiotic and biotic factors on seasonal variation of chlorophyll-a in shallow lakes, water samples were collected semimonthly from February 2009 to January 2010 in Lake Tingtang, and four-day or one-week intervals from May 2008 to April 2009 in Lake Jinghu, respectively. Simultaneously, the abiotic factors including water temperature, pH, dissoloved oxygen, Secchi depth, total nitrogen(TN) and total phosphorus(TP) as well as the densities of rotifers and cladocerans were analyzed. The chlorophyll-a concentration in each water body was measured in two fractions: one filtered through a 25 μm plankton net (Chl.a<25 μm) and another unfiltered (Chl.a). During the monitoring period, the two fractions of chlorophyll-a in two lakes were both positively correlated with water temperature and negatively associated with secchi depth, and they did not show reverse relationship with the densities of rotifers and cladocerans in both lakes. Chl.a and Chl.a<25 μm increased significantly with TP but decreased with TN/TP ratio in Lake Jinghu, while they showed reducing trends with TN and TN/TP ratio in Lake Tingtang. During winter, the rotifer density was negatively associated with Chl.a and Chl.a<25 μm in Lake Jinghu, and the similar relationships between them were found in Lake Tingtang, indicating that the low chlorophyll-a concentration in winter might be partly due to the prey effect of rotifers on phytoplankton. Based on the relative weight of the effect intensity of each correlated factor on the variation of chlorophyll-a concentration, Chl.a and Chl.a<25 μm were both affected mostly by secchi depth. Additionally, the two fractions of chlorophyll-a decreased with an increase in secchi depth. Taken together, the results in this paper implied that Secchi depth might be a simple and convenient index for predicting chlorophyll-a concentration in the small enclosed lakes.