TN : TP ratio and planktivorous fish do not affect nutrient‐chlorophyll relationships in shallow lakes

Abstract

Summary 1. In previous work, phytoplankton regulation in freshwater lakes has been associated with many factors. Among these, the ratio of total nitrogen to total phosphorus (TN : TP) has been widely proposed as an index to identify whether phytoplankton are N‐ or P‐limited. From another point of view, it has been suggested that planktivorous fish can be used to control phytoplankton. 2. Large‐scale investigations of phytoplankton biomass [measured as chlorophyll a, (chl‐a)] were carried out in 45 mid‐lower Yangtze shallow lakes to test hypotheses concerning nutrient limitation (assessed with TN : TP ratios) and phytoplankton control by planktivorous fish. 3. Regression analyses indicated that TP was the primary regulating factor and TN the second regulating factor for both annual and summer phytoplankton chl‐a. In separate nutrient–chl‐a regression analyses for lakes of different TN : TP ratios, TP was also superior to TN in predicting chl‐a at all particular TN : TP ranges and over the entire TN : TP spectrum. Further analyses found that chl‐a : TP was not influenced by TN : TP, while chl‐a : TN was positively and highly correlated to TP : TN. 4. Based on these results, and others in the literature, we argue that the TN : TP ratio is inappropriate as an index to identify limiting nutrients. It is almost impossible to specify a ‘cut‐off’ TN : TP ratio to identify a limiting nutrient for a multi‐species community because optimal N : P ratios vary greatly among phytoplankton species. 5. Lakes with yields of planktivorous fish (silver and bighead carp, the species native to China) >100 kg ha−1 had significantly higher chl‐a and lower Secchi depth than those with yields <100 kg ha−1. TP–chl‐a and TP–Secchi depth relationships are not significantly different between lakes with yields >100 kg ha−1 or <100 kg ha−1. These results indicate that the fish failed to decrease chl‐a yield or enhance ZSD. Therefore, silver carp and bighead carp are not recommended as a biotic agent for phytoplankton control in lake management if the goal is to control the entire phytoplankton and to enhance water quality.

Vulnerabilidade à eutrofização de dois lagos tropicais de climas úmido (Cuba) e semiárido (Brasil)

“…Os limiares da razão N:P têm variações na literatura (WANG et al, 2008), sendo a razão 16:1, apresentada por Redfield (1958), comumente adotada como referência. Segundo Klausmeier et al(2004), essa não é uma razão ótima universal, mas uma média das proporções específicas entre as espécies, de modo que a razão ótima N:P varia entre 8,2 e 45,0.…”

Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

“…(i) log 10 (TP)Àlog 10 (Chl a) relationships do not differ markedly among shallow lakes at least from temperate to subtropical zones, with the regression equation as log 10 (Chl a) = 0.89 (0.73-1.05) log 10 (TP) À 0.38(À0.15 to 0.97) (Canfield, 1983;N€ urnberg, 1996;Brown et al, 2000;Flanagan et al, 2003;Huszar et al, 2006;Wang et al, 2008).…”

Effects of high nitrogen concentrations on the growth of submersed macrophytes at moderate phosphorus concentrations

“…Controversy exists, however, on the effects of increasing N concentrations on submersed macrophytes and the mechanisms behind. First, it is difficult to distinguish the effects of N from those of P due to the close NeP relationship (Downing, 1992;Wang et al, 2008b;Kosten et al, 2009), and the role of N versus P in limiting phytoplankton per se has long been debated (Elser et al, 2007;Lewis and Wurtsbaugh, 2008;Schindler et al, 2008;Conley et al, 2009;Wang et al, 2008b, Wang andWang, 2009). Secondly, some field experiments suggest that periphyton abundance is determined by the abundance of grazers rather than by the concentrations of nutrients (Jones et al, 2002;Jones and Sayer, 2003).…”