Goose-mediated nutrient enrichment and planktonic grazer control in arctic freshwater ponds

A dramatic increase in the breeding population of geese has occurred over the past few decades at Svalbard. This may strongly impact the fragile ecosystems of the Arctic tundra because many of the ultra-oligotrophic freshwater systems experience enrichment from goose feces. We surveyed 21 shallow tundra ponds along a gradient of nutrient enrichment based on exposure to geese. Concentrations of total phosphorus (P) and dissolved inorganic nitrogen (DIN) in the tundra ponds ranged from 2–76 to 2–23 μg l−1 respectively, yet there was no significant increase in phytoplankton biomass (measured as chlorophyll a; range: 0.6–7.3 μg l−1) along the nutrient gradient. This lack of response may be the result of the trophic structure of these ecosystems, which consists of only a two-trophic level food chain with high biomasses of the efficient zooplankton grazer Daphnia in the absence of fish and scarcity of invertebrate predators. Our results indicate that this may cause a highly efficient grazing control of phytoplankton in all ponds, supported by the fact that large fractions of the nutrient pools were bound in zooplankton biomass. The median percentage of Daphnia–N and Daphnia–P content to particulate (sestonic) N and P was 338 and 3009%, respectively, which is extremely high compared to temperate lakes. Our data suggest that Daphnia in shallow arctic ponds is heavily subsidized by major inputs of energy from other food sources (bacteria, benthic biofilm), which may be crucial to the persistence of strong top–down control of pelagic algae by Daphnia.

Nutrient limitation of periphyton growth in arctic lakes in south-west Greenland

This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.Many arctic lakes are oligotrophic systems where phototrophic growth is controlled by nutrient supply. Recent anthropogenic nutrient loading is associated with biological and/or physico-chemical change in several lakes across the arctic. Shifts in nutrient limitation (nitrogen (N), phosphorus (P), or N ? P) and associated effects on the growth and composition of algal communities are commonly reported. The Kangerlussuaq region of south-west Greenland forms a major lake district which is considered to receive little direct anthropogenic disturbance. However, long-range transport of pollutant N is now reaching Greenland, and it was hypothesised that a precipitation gradient from the inland ice sheet margin to the coast might also deliver increased N deposition. In situ nutrient bioassays were deployed in three lakes across the region: ice sheet margin, inland (close to Kangerlussuaq) and the coast (near Sisimiut), to determine nutrient limitation of lakes and investigate any effects of nutrients on periphyton growth and community composition. Nutrient limitation differed amongst lakes: N limitation (ice sheet margin), N and P limitation (inland) and N ? P co-limitation (coast). Factors including variation in N supply, ice phenology, seasonal algal succession, community structure and physical limnology are explored as mechanisms to explain differences amongst lakes. Nutrient limitation of arctic lakes and associated ecological impacts are highly variable, even across small geographic areas. In this highly sensitive region, future environmental change scenarios carry a strong risk of significantly altering nutrient limitation; in turn, potentially severely impacting lake structure and function