Increases in phosphorus at the sediment-water interface may influence the initiation of cyanobacterial blooms in an oligotrophic lake

摘要

Freshwater cyanobacteria have been implicated as both indi-cators and agents of ecosystem change (WETZEL 2001). Glo-eotrichia echinulata is a nuisance cyanobacterium common in meso- to eutrophic systems that has recently begun to bloom in oligo- to mesotrophic lakes across northern New England, USA (CAREY et al. 2008). Between 2002 and 2006, at least 27 outbreaks of G. echinulata were confirmed in oli-gotrophic lakes across Maine and New Hampshire that do not normally experience algal blooms. These blooms are of par- ticular concem because G. echinulata produces the toxin microcystin-LR (CAREY et al. 2007) and can transport a con-siderable amount of phosphorus (P) from the sediment into the water column (ISTvANOVICS et al. 1993). At present, little is known about the mechanisms that have stimulated G. echinulata blooms in oligotrophic systems. We hypothesized that increased P concentrations in lake sedi-ments may be responsible, in part, for G. echinulata blooms in low-nutrient lakes. Many oligotrophic lakes have low con-centrations of P in their water, but high concentrations of P in their sediments (MAASSEN et al. 2005), potentially due to increasing external P loads (WETZEL 2001). Interestingly, many of the lakes in Maine that are now experiencing G. echinulata blooms had been classified as "at risk from new watershed development" (MAINE DEPARTMENT OF ENVI-RONMENTAL SERVICES, unpublished), indicating that those lakes may be subject to increased sediment P. Thus, one hypothesis is that the recent G. echinulata blooms may be occurring in response to increased P concentrations in lake sediment: "eutrophied" sediment in an oligotrophic lake. Phosphorus plays a major role in G. echinulata's complex life cycle. Overwintering akinetes, or resting cells, must absorb a substantial amount of phosphate from the sediment pore water during a germination and growth phase on the lake sediment before recruitment into the water column (PETTERSSON et al. 1993, KARLSSON 2003, KA RLSSON-ELFGREN et al. 2004), which takes -2.5-4 weeks after the initia-tion of germination. Phosphorus absorbed in the sediments fuels planktonic cell division; G. echinulata's P uptake in the water column is thought to be negligible (ISTVANOVICS et al. 1993, TYMOWSKI & DUTHIE 2000). Because of the large pool of sediment P that exists, even in oligotrophic lakes (MAASSEN et al. 2005), and G. echinulata's apparent demand for P from the sediment, we hypothesized that akinetes would exhibit increased germination and recruitment when exposed to increased concentrations of sediment P We used an in situ nutrient-diffusing substrate experiment to test this hypothesis in an oligotrophic lake while simultaneously monitoring G. echinulata recruitment and Pat the sediment-water interface.