Response to Comment: Nitrogen fixation has not offset declines in the Lake 227 nitrogen pool and shows that nitrogen control deserves consideration in aquatic ecosystems

Abstract

We thank Paterson et al. (2011) for their comment on Scott and McCarthy (2010). As stated in our original paper, we agree that Lake 227 has remained eutrophic in response to phosphorus (P) fertilization alone. However, the effect of halting nitrogen (N) fertilization cannot be fully evaluated until the N pool in Lake 227 has reached steady state. The relatively high chlorophyll a (Chl a) and phytoplankton biomasses reported for 2007and 2009(Paterson et al. 2011 were not surprising considering the tremendous variability in these measurements over the entire experimental period. However, these new data do not nullify the decreases in total N (TN) and Chl a from 1990 to 2005, determined from data in Schindler et al. (2008). Additional years of P fertilization at the current rate are needed to determine if decreasing N, but stable P, results in measurable decreases in Chl a or phytoplankton biomass.TN concentrations in Lake 227 have continued to decline, but total P (TP) concentrations have not changed since N fertilization was suspended in 1990. Furthermore, dissolved inorganic N (DIN) : total dissolved P (TDP) and total dissolved N (TDN) : TDP ratios have decreased, and heterocyte abundances have increased since 1990 (Paterson et al. 2011). Paterson et al. (2011 misinterpret the increase in heterocyte abundance as a signal that the phytoplankton have overcome N deficiency. The mere presence of heterocystous cyanobacteria does not indicate that N fixation is meeting phytoplankton N demand (Lewis and Wurtsbaugh 2008). If N-fixing cyanobacteria were supplying N at a rate proportional to P fertilization, TN concentrations would remain constant or increase relative to TP. Instead, N losses from denitrification, burial, or washout have been greater than ecosystem N gains from N fixation, which confirms that fixed N has not accumulated in Lake 227 over a multi-annual timescale (Scott and McCarthy 2010).The energetic costs of N fixation limit phytoplankton primary production and biomass accumulation. Many studies have shown that cyanobacteria differentiating heterocytes and fixing N have lower growth efficiencies and accumulate less biomass per unit N than those assimilating DIN (Rhee and Lederman 1983;Attridge and Rowell 1997;De Nobel et al. 1997). Therefore, the tradeoff associated with the high energetic costs of N fixation, compared with the relatively low energetic costs of DIN assimilation, will yield less biomass when cyanobacteria are fixing N, even at the ecosystem scale.Using a meta-analysis of almost 900 studies on freshwater and marine environments, Elser et al. (2007) showed that the combination of N + P enrichment always yielded more biomass than N or P enrichment alone. The tremendous temporal variability in Chl a and phytoplankton biomasses over the 41-yr experiment in Lake 227 masks this effect. However, when data were grouped by the seven unique combinations in P and N fertilization rates (Schindler et al. 2008;Paterson et al. 2011), average annual Chl a concentrations were strongly proportional to annua...

The dilemma of controlling cultural eutrophication of lakes

“…Lake 227 has a moderate water renewal time (about 4 years); so a high proportion of fixed atmospheric nitrogen is sedimented with algal remains and recycled. Contrary to the unsubstantiated claims of Scott & McCarthy [23], denitrification losses are low. This was the case even when the lake was fertilized with high N : P ratios in the early 1970s [43].…”

“…Many authors have assumed that to be effective, annual nitrogen fixation must account for the entire annual nitrogen requirement for algal production [4,5,8,23]. In lakes, this is not the case [17,18].…”

“…The flawed figure was cited [9,23] as evidence that nitrogen as well as phosphorus must be controlled to reverse eutrophication. More data are available for fertilized ELA lakes than was the case in 1979.…”

“…The average duration of a single study included in the meta-analysis was only 7 days, and the result is not surprising as proximate responses. Some have argued that this result means that reducing nitrogen loading as well as phosphorus will result in more rapid declines in eutrophication in whole lakes [9,23]. Recently, a 7-day experiment in mesocosms of various sizes was compared with the above-mentioned meta-analyses, concluding erroneously that short-term ecosystem results can therefore be scaled to whole lakes [24], even though no whole-ecosystem measurements were included, and ecosystem processes such as nitrogen fixation and recycling were ignored.…”

Nitrogen Constrains the Growth of Late Summer Cyanobacterial Blooms in Lake Erie

“…Recently, regulating both N and P to control eutrophication has had support [9][10][11] while others are in favor of P only control [8,39,40]. N-fixation in Lake Erie was documented during the Anabaena blooms of 2010 and 2011 (Bade unpublished data), but it is unknown if N-fixation can offset N deficiencies in small lakes [41,42] and for Lake Erie, a large lake.…”