Nitrogen and phosphorus inputs control phytoplankton growth in eutrophic Lake Taihu, China

Abstract

Lake Taihu (Taihu) is the third largest freshwater lake in China and an important drinking water, fishing, and tourism resource for Jiangsu Province. Recent toxic cyanobacterial blooms caused by excessive human nutrient loading have focused attention on arresting blooms and restoring the lake to acceptable water quality conditions by reducing nutrient inputs. Field sampling and in situ nutrient enrichment bioassays were conducted to determine seasonal patterns of nutrient limitation and nutrient thresholds for phytoplankton growth. The TN : TP and TDN : TDP mass ratios in the ambient water showed high seasonal variation and changed from 33-80 : 1 and 52-212 : 1, respectively, in winter and spring, and both declined to below 20 : 1 in summer. In spring and winter, total phytoplankton biomass and growth rates increased significantly with additions of P, with no primary effects from N, suggesting P limitation of phytoplankton growth. During the summer and fall bloom periods, however, N additions alone revealed a significant positive effect on phytoplankton growth, and P additions only stimulated phytoplankton growth once N had been added, suggesting that N was the primary limiting nutrient, with P being a secondarily limiting nutrient. When P enrichment was $ 0.20 mg P L 21 and N enrichment $ 0.80 mg N L 21 , growth of the toxin-producing, dominant bloom-forming cyanobacteria Microcystis spp. was not nutrient limited. This study suggests that availability of N during the summer is a key growth-limiting factor for the proliferation and maintenance of toxic Microcystis spp. blooms. Therefore, although P load reduction is important, N load reduction is essential for controlling the magnitude and duration of algal booms in Taihu.

Critical nutrient thresholds needed to control eutrophication and synergistic interactions between phosphorus and different nitrogen sources

“…The most common way of adding nutrient is adding a finite supply of dissolved nutrients as the setting concentrations at the beginning of the experiment and adding no more nutrients during the process of phytoplankton incubation (Domingues et al 2011;Ma et al 2015a, b;Paerl et al 2011;Xu et al 2010Xu et al , 2015. However, there are some problems with this approach.…”

“…Nutrient threshold is defined as the critical levels of N and P that control an abrupt change or regime shift such as sudden and protracted dominance by phytoplankton (Xu et al 2014). Previous studies have shown that eutrophication thresholds of total phosphorus (TP) for freshwaters are from 0.02 to 0.10 mg P L −1 and of total nitrogen (TN) are from 0.50 to 1.00 mg N L −1 (Xu et al 2010). Abundant evidence has proven that understanding and applying ecological thresholds is the key to successful environmental management (Groffman et al 2006).…”

“…In addition, ammonium (NH 4 + ) recycling may be equally rapid with phosphate (PO 4 3− ) recycling ). However, in most studies, the eutrophication threshold of N referred to the threshold of nitrate-nitrogen (NO 3 -N), and little information is available about the threshold range of ammonium-nitrogen (NH 4 -N) Xu et al 2010Xu et al , 2014. What is more, the nutrient colimitation bioassays were generally conducted with phosphorus and nitrate-nitrogen addition and little is known about the colimitation and synergistic interactions among ammonium-nitrogen, nitrate-nitrogen, and phosphorus.…”

“…The nutrient threshold bioassay experiment was conducted in July 2015 to determine the nutrient threshold levels of SRP, NO 3 -N, and NH 4 -N. Chlorophyll a (Chl a) concentrations were used to estimate phytoplankton biomass (Lewis et al 2011;Paerl et al 2011;Xu et al 2010). The upper limit of these nutrients needed to control phytoplankton growth and bloom formation (Chl a concentration exceeding 20 μg L −1 ) could be established using this approach.…”

“…Response curves were established, and the change points on the curves were estimated as the eutrophication threshold Xu et al 2010Xu et al , 2014. The changepoint analyzer 2.3 (Taylor Enterprises, Inc.) was used for examining the change plots on the response curves.…”