Phytoplankton nutrient limitation in a polymictic eutrophic lake: community versus species-specific responses

Comment on "Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand" by Morgenstern et al. (2015)

“…Such concentrations are generally below levels at which PO 3− 4 concentrations have the potential to suppress phytoplankton growth rates (∼ 0.003 mg P L −1 ; Reynolds, 2006) and, depending on the availability of other resources such as light and N, these concentrations have the potential to limit phytoplankton biomass accumulation. Indeed, phytoplankton biomass accumulation in the lake has been shown experimentally to be limited at times by P, either in isolation or in conjunction with N (Burger et al, 2007;Smith et al, 2016). These observations are consistent with the view more generally that co-limitation of phytoplankton biomass accumulation by both N and P is commonplace in freshwater ecosystems (Elser et al, 2007).…”

“…This conclusion by the authors contradicts the current approach to managing water quality in Lake Rotorua that is based on a strategy of dual control of N and P (BoPRC, 2004;BoPRC et al, 2009;Burns et al, 2009), founded on the results of research conducted on the lake over several decades (e.g. Fish, 1975;Rutherford et al, 1989;Burger et al, 2007). This strategy of dual nutrient control is supported by a multi-million dollar publicly funded restoration programme, with statutory instruments now in place to help achieve load reduction targets (Parliamentary Commissioner for the Environment, 2006;Burns et al, 2009).…”

“…see Burns et al, 2009). In addition, the potential for either N, P, or N and P to limit phytoplankton biomass accumulation at times in Lake Rotorua (Burger et al, 2007) means that a focus on controlling both nutrients is expected to be more efficient that controlling only one nutrient (cf. Lewis et al, 2011).…”

Assessing nutrient limitation in a subtropical reservoir

“…Another key reason for the differential impacts of nutrient limitation is that the growth response of phytoplankton is likely to be related to the community composition and competitive ability of species to make use of the nutrient inputs (Hecky and Kilham 1988, Mitrovic et al 2001, Burger et al 2007. Rhee (1978) demonstrated that the optimal cellular N:P may be species-specific (i.e., growth of different phytoplankton species may be limited by different nutrients).…”

Modelling the response of a highly eutrophic lake to reductions in external and internal nutrient loading

“…It appears to be counterintuitive to reduce N loading alone, considering the risk of promoting N-fixing cyanobacteria in an ecosystem which is already eutrophic. However, while reducing P loading alone has been shown to result in major reductions in chl a concentrations in a series of Danish lakes (Jeppesen et al 2007), it may be difficult to achieve P-limitation in many lakes of the central North Island of New Zealand, which have a tendency to be N-limited or co-limited by both N and P because of P-rich volcanic soils (White et al 1985;Burger et al 2007b;Abell et al 2010). This trend towards N-limitation may be exacerbated by the dominance of cyanobacteria with the ability for luxury P uptake (Reynolds 2006).…”