Nitrogen and phosphorus limitation and the management of small productive lakes

Abstract

Many inland waters are enriched with nutrients, causing deleterious effects to their ecology and the benefits they provide for society, but their effective management first requires identification of the nutrient(s) that limit algal production. Concentrations of nutrients and chlorophyll a (Chl-a) were used to assess nutrient limitation seasonally at 17 meres over 2 time periods: historic (2005-2009; 1995-1998 at one site) and contemporary (2014-2018). Different approaches were used to assess nutrient limitation because they reflect different aspects of nutrient availability and their conversion into biomass. In the historic period, 3 meres were phosphorus (P) limited, 3 nitrogen (N) limited, 5 co-limited; the remaining 6 meres were not nutrient limited. For this period, ecological status assessed using phytoplankton Chl-a was only at good or high ecological status (sensu the Water Framework Directive) at 2 sites. The contemporary period was slightly improved, with 4 sites at good status. At the sites that failed to meet good ecological status, the required reduction in P concentration was least in P-limited sites and, conversely, the reduction in N was least in N-limited sites, suggesting that remediation by nutrient reduction would be most efficient if it was targeted using site-specific information. Even in primarily P-limited sites, once input of P has been reduced, further ecological benefit of reducing N at targeted sites should be explored.

What Colin Reynolds could tell us about nutrient limitation, N:P ratios and eutrophication control

“…Therefore, nonlimiting situations need to be excluded when drawing conclusions for nutrient limitation from N:P ratios or from growth responses in nutrient enrichment experiments (as done, e.g. by Maberly et al, 2020). Chlorophyll-a concentration [µg/L] Fig.…”

“…In this model, competition for limiting nutrients selects for effective nutrient uptake (requiring high N:P ratios) while exponential growth requires more P and thus, depending on the limiting resource, the model of these authors predicts N:P ratios that range from 8.2 (by mass) at saturating resource levels up to 45 under P limitation. From empirical studies, others conclude a range of 16-30 (by mass) outside of which N or P limitation are likely (see discussion in Dolman & Wiedner, 2015) or a molar ratio of \ 10 for N-limitation being likely and [ 20 for a probable P limitation (Maberly et al, 2020). Therefore within this range there is no 'universally optimal value'; rather, the N:P ratio of a phytoplankton community at a given point in time is an average, resulting from the composition of species with 'different structural N:P ratios' (Klausmeier et al, 2004, p. 173).…”

“…In practice, the molar range of 10-20 outside of which N, respectively, P to be likely to be limiting is frequently used (e.g. Maberly et al, 2020). (ii) A further complication is the above-mentioned problem of separating phytoplankton from other seston particles to analyse their constituents.…”

“…Factoring light availability (photon flux density) into an experimental design and extrapolating the experimental results to the situation in the waterbody is challenging and rarely done. Kolzau et al (2014) give an example, and Maberly et al (2020) use the approach suggested by Reynolds (1992) to calculate light limitation in lake by stating that at in situ mean epilimnetic light intensities above 20 lmol m -2 s -1 photosynthesis is likely to be light-saturated. To assess whether the growth responses in the culture vessels are relevant to the field situation, both research groups also use information about the concentrations of dissolved P (SRP) and N (DIN), as well as Chl.a concentrations in relation to TP and TN in the waterbody.…”

“…It is increasingly recognised that in many waterbodies summer phytoplankton is co-limited by P and N (e.g. Elser et al, 2007;Kolzau et al, 2014;Shatwell & Köhler, 2019;Andersen et al, 2020;Maberly et al, 2020), and N:P ratios are also quoted to argue for the need to reduce N loads.…”