The effects of grazing by the snail, Lymnaea elodes, on benthic N2 fixation and primary production in oligotrophic, arctic lakes
Limnol. Oceanogr., 52(6), 2007, 2398-2409 | DOI: 10.4319/lo.2007.52.6.2398
ABSTRACT: This study assessed whether grazing by the snail, Lymnaea elodes, limits benthic dinitrogen (N2) fixation and primary production in nitrogen (N)-limited oligotrophic lakes near Toolik Field Station on the North Slope of Alaska. We also tested whether snail excretion increased N and the ratio of N and phosphorus (P) supply ratio to benthic algae, which could indirectly affect production and the N2 fixation rate. We performed in situ, randomized block experiments in two lakes in 3 years in which snail density was manipulated and compared to open cage controls. Snails significantly decreased areal rates of N2 fixation in both lakes in all years (p < 0.05), but did not appear to cause a reduction in cyanobacterial abundance or filament size (p > 0.05). Snails did not significantly affect measures of benthic production, including gross primary production, respiration, net ecosystem production, and chlorophyll biomass (p > 0.05). Snail-induced declines in N2 fixation probably did not result from snail excretion. The molar N: P excretion ratio of ammonium (NH4+) and phosphate (PO4+) was very low (4.8), indicating that snails likely exacerbated N limitation, a response that would tend to favor enhanced rather than reduced N2 fixation. Furthermore, the excretion rate of N-NH4+ was several orders of magnitude lower than the N2 fixation rate (0.002-0.02 mg N m-2 day-1 vs. 0.1-0.4 mg N m-2 day-1, respectively) and met almost none (<<1%) of the N demand by primary producers. Although the mechanism by which Lymnaea elodes caused a decline in N2 fixation is unknown, the effect was small, and accounted for a reduction of N inputs of only 0.12 mg N m-2 summer-1 or by 0.85-1.8% at ambient snail densities. Because N2 fixation is a new N input able to support new production, this effect may be important across long time scales or where densities of L. elodes are higher.