Control mechanisms of arctic lake ecosystems: a limnocorral experiment

Abstract

To assess the potential impact of human exploitation on arctic lakes and to determine how these ecosystems are regulated we initated a limnocorral experiment in Toolik Lake, Alaska, in the summer of 1983. The limnocorrals were 5 m in diameter and from 5-6 m in depth and were open to the sediments. In 1983 four limnocorrals were deployed in an isolated bay of Toolik Lake within a cross-classified treatment regime of high and low inorganic nitrogen and phosphorus additions and high and low free swimming fish additions. The objective of the nutrient addition was to stimulate phytoplankton growth and determine the extent to which increased plant production was passed through pelagic and benthic food chains. The objective of the fish addition was to determine the impact of fish predation on largebodied zooplankton, especially the zooplanktivorous copepod Heterocope, then to study the effect of altered Heterocope densities on small-bodied zooplankton species population dynamics. In 1984 two more limnocorrals were deployed, one a low fish, 1 x nutrient addition treatment and the other a no fish, no nutrient treatment. The fish manipulation was changed to confining several fish in cages with the cages held in corrals for varying lengths of time.The addition of inorganic nitrogen and phosphorus dramatically increased phytoplankton productivity. This increase in algal biomass and production greatly altered the light environment and water quality in the nutrient treated limnocorrals. The secchi disk depth in the nutrient treated limnocorrals declined each summer reaching as low as 1 m in 1985. Both oxygen content and pH increased in the nutrient treatment corrals. Corrals not receiving nutrient additions remained near lake concentrations for most water quality parameters. While phytoplankton biomass was stimulated in 1983 phytoplankton growth was not sufficient to draw down all the nitrogen and phosphorus added and these nutrients reached high levels in the last half of the summer. In 1984 phosphorus remained above 20 g in the nutrient-treated corrals but ammonia dropped to reference levels by day 25. In 1985 both nutrient concentrations rapidly declined to reference levels.Most pelagic components responded to the nutrient additions. Microbial production was stimulated in the nutrient treated limnocorrals and bacterial population sizes built up to nearly 8-10 times those of the reference corrals. However, microheterotrophs soon increased in abundance and apparently grazed down bacteria to reference levels. Phytoplankton population density, as estimated by chlorophyll a determinations, increased dramatically with nutrient addition such that each year the phytoplankton densities were higher than before. Primary productivity was also stimulated and appeared not to be light 144 limited even when phytoplankton densities rose to high levels. In the first two years of the experiment zooplankton densities were little altered by the increased phytoplankton densities. However, by 1985 daphnid densities were quite a bit higher in the ...

Phosphorus limitation of bacterial growth in high Arctic lakes and ponds

“…However, previous full-water fertilization experiments in Toolik Lake (Hobbie and Helfrich III, 1988;O'Brien et al, 1992;Hobbie et al, 1999) as well as Arctic ponds and streams (Douglas and Smol, 2000;Peterson et al, 1985) have revealed that microbial food webs of Arctic freshwaters respond in the same way as temperate lakes to P amendments; i.e., algal biomass increases dramatically, often paralleled by an increase in bacterial biomass. Still, the observed increases in bacterial biomass have mainly been interpreted as an indirect effect of stimulated primary production and associated input of organic C for subsequent bacterial utilization (Hobbie et al, 1999;O'Brien et al, 1992) rather than direct nutrient limitation of bacterial cells. The former interpretation is supported by a separate study where the direct effects of extraneous DOC on microbial food webs were studied in full-water Toolik Lake mesocosms (Hobbie et al, 1996).…”

“…This situation typically produces ultra-oligotrophic freshwater ecosystems due to low inputs of nutrients and organic carbon from terrestrial surroundings (Pienitz et al, 1997a, b). Low internal production is also a direct effect of annual temperatures that seldom exceed 5°C, but fertilization experiments have nevertheless shown that Arctic lakes can support high primary production in spite of the typically year-round low temperatures (O'Brien et al, 1992;Douglas and Smol, 2000). Another constraint on aquatic primary productivity in this region is the persistent ice cover that can cover lakes for more than 11 months per year.…”

Change of microplankton community structure in response to fertilization of an arctic lake

“…Previous studies at the Arctic LTER site included fertilization and fish density manipulations of limnocorrals (O'Brien et al ., 1992) and fertilization of one-half of a naturally oligotrophic lake (Lake N-2) which had been divided by a plastic curtain and from which most fish had been removed (O'Brien et al ., in preparation) . The 'bottom-up' responses to fertilization included increases in nutrient pools and increased biomass of phytoplankton, nanoflagellates, benthic algae and submerged vegetation, benthic insects, and crustacean zooplankton (after a 1-2 year lag) .…”

“…The 'bottom-up' responses to fertilization included increases in nutrient pools and increased biomass of phytoplankton, nanoflagellates, benthic algae and submerged vegetation, benthic insects, and crustacean zooplankton (after a 1-2 year lag) . There was also evidence of top-down regulation, manifested in apparent selection favoring small zooplankton when fish were present (O'Brien et al, 1992) . Microplankton were not studied in either the limnocorrals or in lake N-2 prior to 1989 (the 3rd year offertilization of N-2) .…”

Ratio-dependent response of a temperate Australian estuarine system to sustained nitrogen loading

“…McNaughton 1989) and lakes (e.g. Arditi et al 1991;Hansson 1992;O'Brien et al 1992;Persson et al 1992), comparing trophic structure and/ or trophic level biomass across natural gradients of productivity (e.g. Hansson 1992;Persson et al 1992) or measuring the response of a community to nutrient enrichment (e.g.…”