Identifying the Mechanisms behind the Positive Feedback Loop between Nitrogen Cycling and Algal Blooms in a Shallow Eutrophic Lake

Yu Yao,Huaji Liu,Dujun Li,Ruiming Han,Limin Zhang

doi:10.3390/w13040524

Abstract

Algal blooms have increased in frequency, intensity, and duration in response to nitrogen (N) cycling in freshwater ecosystems. We conducted a high-resolution sedimentary study of N transformation and its associated microbial activity in Lake Taihu to assess the accumulation rates of the different N fractions in response to algal blooms, aiming to understand the mechanisms of N cycling in lacustrine environments. Downcore nitrification and denitrification processes were measured simultaneously in situ via diffusive gradients in thin-films technique, peeper, and microelectrode devices in a region of intensified algal blooms of shallow lake. The decomposition of different biomasses of algal blooms did not change the main controlling factor on different N fractions in profundal sediment. However, the decomposition of different algal biomasses led to significant differences in the nitrification and denitrification processes at the sediment–water interface (SWI). Low algal biomasses facilitated the classic process of N cycling, with the balanced interaction between nitrification and denitrification. However, the extreme hypoxia under high algal biomasses significantly limited nitrification at the SWI, which in turn, restricted denitrification due to the lack of available substrates. Our high-resolution results combined with estimates of apparent diffusion fluxes of the different N fractions inferred that the lack of substrates for denitrification was the main factor influencing the positive feedback loop between N and eutrophication in freshwater ecosystems. Moreover, this positive feedback can become irreversible without technological intervention.

Highlights

During the last few centuries, the over-enrichment of anthropogenic nutrients has enhanced eutrophication and increased the magnitude and frequency of algal blooms in aquatic ecosystems
The basic water quality characteristics are outlined in Table 1.The diffusion coefficients of labile fractions were selected to ensure the accurate evaluation of the diffusive gradients in thin-films technique (DGT) technique according to the temperature of the sampling sites
We investigated the impacts of sediment algal decomposition on the variation of physicochemical properties in the overlying water of

Summary

Introduction

During the last few centuries, the over-enrichment of anthropogenic nutrients has enhanced eutrophication and increased the magnitude and frequency of algal blooms in aquatic ecosystems. Algal blooms have directly affected drinking water supplies, which has further impacted aquaculture and tourism development [1,2,3]. Lake eutrophication and algal blooms occur in response to overloaded nutrient concentrations in the water column— nitrogen (N) [4,5]. Continuous N accumulation in lakes alters sediment functioning from its sink to source, which further influences the occurrence of algal blooms [6,7]. Several investigations have identified the “positive feedback cycle” between the labile N fractions, microhabitat, dissolved organic matter (DOM), and algal blooms [8,9]

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