Growth rate of alpine phytoplankton assemblages from contrasting watersheds and N‐deposition regimes exposed to nitrogen and phosphorus enrichments

Abstract

High mountain lakes are a network of sentinels, sensitive to any events occurring within their waterbodies, their surrounding catchments and their airsheds. By modifying nutrient balance and availability in water, both local and global changes are expected to alter primary productivity and to trigger strong ecological impacts in these ecosystems. Predicting ecological trajectories under future change is a key challenge for both scientists and conservation managers. French alpine lakes, in the most southern and western part of the European Alps, have received surprisingly little attention to date. In this article, we address how variations in nitrogen (N) and phosphorus (P) supply are likely to impact the area's phytoplankton growth. We performed N and/or P enrichment microcosm experiments under controlled conditions on 12 phytoplankton assemblages sampled during the summer 2016 in four French alpine lakes with contrasting catchments and N‐deposition regimes. The nutrients limiting phytoplankton growth varied according to the nutrient stoichiometry of the lake water. In the lakes exposed to high N‐deposition rates (≈700 kg N km–2 year–1), the water contained more N than P and phytoplankton growth in microcosms was either limited by P or not limited by either N or P. In the lakes exposed to low N‐deposition rates (≈500 kg N km–2 year–1), N availability relative to P was lower in the lake with vegetated catchment than in the lake with rocky catchment, resulting in a switch from P to NP colimitation of the phytoplankton in microcosms. Our data clearly indicate that French alpine lakes do not host the same diversity and structure of phytoplankton communities and that phytoplankton composition influenced phytoplankton growth in microcosms. First, we show that nonmotile colonial chlorophytes appeared in late summer assemblages, with lower growth rates but P‐storage abilities. Second, our findings indicate that the growth of phytoplankton assemblages dominated by diatoms was increasingly limited by silica (SiO2) throughout the summer, along with a 70% decrease in SiO2 concentration in lake water. The forecast global changes in the French Alps should increase phytoplankton growth in most high mountain lakes where P is the main limiting nutrient, before NP colimitation. These changes are likely to be of lesser extent in lakes with large vegetated catchments in the northern area with lower N‐deposition rates and of greater extent in the southern area with higher N‐deposition rates and future P‐deposition rates. By investigating the relationship between nutrient availability, phytoplankton composition and phytoplankton growth rate, this experimental laboratory microcosm study will help interpret current multifactorial data from in situ monitoring networks in the Alps. It will also be helpful to develop models to better predict the sentinel lake responses to local and global changes.

Effects of catchment area and nutrient deposition regime on phytoplankton functionality in alpine lakes

“…The C/N ratios (7 < C/N < 13) indicated that the POM was mainly of phytoplankton origin, with a low fraction of terrestrial organic matter (C/N >> 10) (Gasiorowski & Sienkiewicz, 2013). The DIN:SRP ratios above the Redfield ratio of 7:1 on a mass basis (80 ± 20 < DIN:SRP < 367 ± 80) indicated that P was the main limiting nutrient in all lakes, as shown in a previous experimental study (Jacquemin et al, 2018).…”

“…They were poorly productive, with a trophic state ranging from ultra-oligotrophic to oligotrophic. As the studied lakes have been shown to be mainly limited by P in a previous experimental study (Jacquemin et al, 2018), the southern lakes were characterized by higher phytoplankton biomass that likely reflected higher P-deposition regime. The almost twice as high dissolved inorganic nitrogen (DIN) concentrations measured in these lakes was consistent with their exposition to almost twice as high N-deposition rates.…”

Microbial consortia in an ice‐covered high‐altitude lake impacted by additions of dissolved organic carbon and nutrients

“…The catchment lithology is mainly metamorphic rock (blue Shales). This lake has been described as oligotrophic by Jacquemin et al (2018), with strong limitation of phytoplankton by N and P. The lake is ice-covered for 7-8 months, from October to June.…”

“…For the nutrient addition, N was added as NH 4 NO 3 to increase N concentrations to 335 µg/L and P was added as KH 2 PO 4 to increase P concentrations to 35 µg/L. The aim was to avoid potential nutrient limitation throughout this experiment, while remaining consistent with natural concentrations of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) (Jacquemin et al, 2018). The 10℃ temperature was chosen in order to mimic the conditions prevailing at the end of the ablation phase (a surface temperature of 2.79℃ was measured on 9 June 2016 in Lake Cordes, subsequently reaching 10.27℃ on 24 June 2016, personal data).…”

“…Then, water from each polycarbonate plastic bottle was manually homogenised and transferred to the 72 microcosms. Each microcosm consisted of a 500-ml sterilised transparent polycarbonate plastic bottle (Nalgene ® ) filled with 350 ml of sampled water to allow gas exchanges, according to the protocol used by Jacquemin et al (2018).…”