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

Abstract

This study assessed whether grazing by the snail, Lymnaea elodes, limits benthic dinitrogen (N 2 ) 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 N 2 fixation rate. We performed in situ, randomizedblock 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 N 2 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 N 2 fixation probably did not result from snail excretion. The molar N : P excretion ratio of ammonium (NH z 4 ) and phosphate (PO z 4 ) was very low (4.8), indicating that snails likely exacerbated N limitation, a response that would tend to favor enhanced rather than reduced N 2 fixation. Furthermore, the excretion rate of N-NH z 4 was several orders of magnitude lower than the N 2 fixation rate (0.002-0.02 mg N m 22 day 21 vs. 0.1-0.4 mg N m 22 day 21 , respectively) and met almost none (,,1%) of the N demand by primary producers. Although the mechanism by which Lymnaea elodes caused a decline in N 2 fixation is unknown, the effect was small, and accounted for a reduction of N inputs of only 0.12 mg N m 22 summer 21 or by 0.85-1.8% at ambient snail densities. Because N 2 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.

Epipelic and pelagic primary production in Alaskan Arctic lakes of varying depth

“…However, high spatiotemporal variability and limited sampling reduced our ability to detect any statistically significant differences among lake classes. Our mean values of epipelic chl a are similar to previously reported concentrations of 332-575 mg m -2 in Arctic Foothill lakes (Whalen et al, 2006;Gettel et al, 2007). However, they are generally higher than the 19-409 mg chl a m -2 reported for subarctic epipelon (Sorsa, 1976;Björk-Ramberg, 1983;Björk-Ramberg & Å nell, 1985;Hansson, 1992;Liboriussen & Jeppesen, 2003) and the 20-379 mg chl a m -2 observed in algal mats of ponds in the high arctic (Bonilla et al, 2005;Rautio & Vincent, 2007).…”

“…However, they are generally higher than the 19-409 mg chl a m -2 reported for subarctic epipelon (Sorsa, 1976;Björk-Ramberg, 1983;Björk-Ramberg & Å nell, 1985;Hansson, 1992;Liboriussen & Jeppesen, 2003) and the 20-379 mg chl a m -2 observed in algal mats of ponds in the high arctic (Bonilla et al, 2005;Rautio & Vincent, 2007). High epipelic chl a concentrations here and in other Arctic Foothill lakes relate at least in part to sampling methodology, as the 2 cm sediment sample depth in Arctic Foothill studies by Whalen et al (2006) and Gettel et al (2007) exceeds the B1 cm common to many prior reports. Viable pigments and live algae have frequently been reported to several cm below the sediment surface (Stanley, 1976;Cariou-LeGall & Blanchard, 1995), including sediments below the zone of active photosynthesis (SandJensen & Borum, 1991).…”

Microphytobenthos in Shallow Arctic Lakes: Fine-Scale Depth Distribution of Chlorophylla, Radiocarbon Assimilation, Irradiance, and Dissolved O2

“…for the surficial 1 cm of sediment are roughly half or less of previously reported values of 8-69 g cm ‫2מ‬ for the 2 cm depth increment (Whalen et al, 2006(Whalen et al, , 2008Gettel et al, 2007) in other Arctic Foothills lakes and are similar to concentrations of 8-38 g cm…”

Methane efflux and oxidation, and use of methane-derived carbon by larval Chironomini, in arctic lake sediments

“…Internally, the caps had central bevels leading to sampling port fittings guarded by septa. Sealed cores were arranged around a central shaft which supported magnets that rotated at one revolution per minute (modified from Gettel et al ). This apparatus was designed to prevent chemical stratification in the overlying water and maintain a gas‐water equilibrium within the cores, while not disturbing the sediment–water interface (Hershey et al ).…”