The effects of grazing by the snail, Lymnaea elodes, on benthic N2 fixation and primary production in oligotrophic, arctic lakes

Gettel, Gretchen M., Anne E. Giblin, Robert W. Howarth

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.

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