Surface sediment diatom assemblages and epilimnetic total phosphorus in large, shallow lakes of the Yangtze floodplain: their relationships and implications for assessing long‐term eutrophication

Abstract

1. The Yangtze floodplain (SE China) is characterized by a number of large shallow lakes, many of which have undergone eutrophication due to the intensification of agriculture and urban growth over recent decades. As monitoring data are limited and in order to determine lake baseline nutrient concentrations, 49 lakes were sampled, covering a total phosphorus (TP) gradient (c. 30-550 lg L )1 ) to develop a diatom-based inference model. 2. There are three dominant diatom assemblages in these shallow lakes with a marked change in assemblage structure near the boundary between eutrophic and hypereutrophic nutrient levels (as indicated by their TP value). Canonical correspondence analysis indicated that TP was the most important and significant variable in explaining the diatom distributions, independently accounting for 9.5% variance of diatoms. 3. Forty-three lakes were used to generate a transfer function using weighted averaging (WA) with inverse deshrinking. This model had low predictive error (root mean squared error of prediction; RMSEP jack = 0.12) and a high coefficient of prediction (R 2 jack = 0.82), comparable with regional TP models elsewhere. The good performance of this TP model may reflect the low abundance of benthic diatom species which are commonly regarded as the main error source in European shallow lake WA models. 4. The WA model was used to reconstruct the past-TP concentrations for Taibai Lake, a shallow hypereutrophic lake in Hubei province. The results showed that TP concentration varied slightly (43-62 lg L )1 ) prior to the 1920s, indicating an eutrophic state since the 1800s. A period of sustained eutrophication occurred after 1950, because of the development of agriculture, reflecting by maximum values of Aulacoseira alpigena and increased abundance of Cyclotella meneghiniana, C. atomus and Cyclostephanos dubius. The steep increase in nutrient concentration after 1970 was related to the overuse of chemical fertilizer and fish farming in the catchment. 5. The shift in fossil diatoms from epiphytic to planktonic forms in the lake sediment core during 1950-70 provides useful information on the transformation from macrophytedominated to alga-dominated states. It is plausible that the TP concentration of 80-110 lg L )1 observed in this study is the critical range for switching between the two stable states in the lake. 1273 6. The regional diatom-TP model developed in this study allows, therefore, the possibility of reconstructing historical background nutrient concentrations in lakes. It will provide an indication of the onset and development of eutrophication at any site. This is particularly important for the many lakes in the Yangtze floodplain where information about historical changes in water quality is lacking.

Historical trophic evolution resulting from changes in climate and ecosystem in Lake Taihu and seven other lakes, China

“…It was obvious that the historical trophic status of the lakes along the middleÀlower reaches of the Yangtze River were already at an intermediate-eutrophic level at the beginning of the 200-yr period, which was in accordance with diatom-inferred TP studies in the same region (Yang et al 2008). Theoretically, the reasons for high background concentrations of nutrients are related to their morphological characteristics.…”

“…By the end of the twentieth century, most of the lakes in the middleÀlower reaches of the Yangtze River had degraded to mesotrophic or eutrophic status (Qin 2002;Yang et al 2008). Much research has been conducted on lake eutrophication in different regions (Perrow & Davy 2002;Qin 2002;Phillips et al 2005).…”

Spatiotemporal variations of Chinese lakes and their driving mechanisms since the Last Glacial Maximum: A review and synthesis of lacustrine sediment archives

“…However, for the past 50 a, the continuously deteriorating eutrophication has been unprecedented, possibly due to the retention of water in response to increasing occurrences of blocked river systems (by man-made dams and water reclamation), as well as the reduced self-purification abilities of lakes as a result of prolonged residence times of nutrients. Indeed, a study of nutrient changes in Lake Longgan on centennial scales revealed that climatic changes serve as the main reason for changes in the nutrients before 1950, after which human activities played a key role, accounting for 60% and 57% of the total TP variations in sediment and in lake water, respectively [98].…”

Spatio-Temporal Response of Sedimentary Diatoms to Water Level in a Shallow Lake

“…To emphasize the impact of human activities on the response of diatom assemblages in surface sediments to water levels, the diatoms of summer samples were identified since supplementary water flowed into the lake from the reservoir in the upper reach between spring and summer. Ordination methods were used to determine the effect of environmental factors on sedimentary diatom distribution (Yang et al, 2008). Detrended correspondence analysis (DCA) was used to estimate the gradient length of the first axis of the diatom distribution.…”