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

Abstract

We compared on eight dates during the icefree period physicochemical properties and rates of phytoplankton and epipelic primary production in six arctic lakes dominated by soft bottom substrate. Lakes were classified as shallow (z \ 2.5 m), intermediate in depth (2.5 m \ z \ 4.5 m), and deep (z [ 4.5 m), with each depth category represented by two lakes. Although shallow lakes circulated freely and intermediate and deep lakes stratified thermally for the entire summer, dissolved oxygen concentrations were always [70% of saturation values. Soluble reactive phosphorus and dissolved inorganic nitrogen (DIN = NO 3 --N + NH 4 + -N) were consistently below the detection limit (0.05 lmol l -1 ) in five lakes. However, one lake shallow lake (GTH 99) periodically showed elevated values of DIN (17 lmol l -1 ), total-P (0.29 lmol l -1 ), and total-N (33 lmol l -1 ), suggesting wind-generated sediment resuspension. Due to increased nutrient availability or entrainment of microphytobenthos, GTH 99 showed the highest average volume-based values of phytoplankton chlorophyll a (chl a) and primary production, which for the six lakes ranged from 1.0 to 2.9 lg l -1 and 0.7-3.8 lmol C l -1 day -1 .Overall, however, increased z resulted in increased area-based values of phytoplankton chl a and primary production, with mean values for the three lake classes ranging from 3.6 to 6.1 mg chl a m -2 and 3.2-5.8 mmol C m -2 day -1 . Average values of epipelic chl a ranged from 131 to 549 mg m -2 for the three depth classes, but levels were not significantly different due to high spatial variability. However, average epipelic primary production was significantly higher in shallow lakes (12.2 mmol C m -2 day -1 ) than in intermediate and deep lakes (3.4 and 2.4 mmol C m -2 day -1 ). Total primary production (6.7-15.4 mmol C m -2 day -1 ) and percent contribution of the epipelon (31-66%) were inversely related to mean depth, such that values for both variables were significantly higher in shallow lakes than in intermediate or deep lakes.

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

“…Our volumetric phytoplankton chl a concentrations of 0.6-2.3 g L ‫1מ‬ are similar to earlier reports from surveys in Arctic Foothills lakes 290 / ARCTIC, ANTARCTIC, AND ALPINE RESEARCH (Kling et al, 1992;Levine and Whalen, 2001;LaPerriere et al, 2003;Whalen et al, 2008) and other regions of the Arctic (Pienitz et al, 1997a(Pienitz et al, , 1997bHamilton et al, 2001;Lim et al, 2001;Michelutti et al, 2002), which often show values around 1-2 g L…”

“…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…”

“…However, sediments experience reduced incident irradiance relative to the overlying water and show attenuation coefficients higher by over three orders of magnitude. Decreased irradiance at the sediment surface and a vertically narrow euphotic zone may be compensated by the increased abundance of algae in the sediment and a horizontally expansive region of sediment surface exposed to light (89-100% in the study lakes) in shallow Arctic lakes and ponds, as whole lake primary production in this region is often dominated by the microphytobenthos (Ramlal et al, 1994;Stanley, 1976;Whalen et al, 2008).…”

“…We recently showed that microphytobenthos account for as much as 77% of total primary production in lakes located in the Arctic Foothills province of Alaska (Whalen et al, 2008), where the importance of benthic phototrophs as a food resource to multiple levels of limnetic and benthic consumers has been previously demonstrated (Sierszen et al, 2003). A specific aim of this study was to expand our earlier effort to understand the role of benthic microalgae in shallow Arctic lake productivity by providing insight into physicochemical gradients and structural and functional aspects of the benthic algal assemblage at vertical scales relevant to community function.…”

An improved coverslip method for investigating epipelic diatoms

“…In freshwaters, epipelic diatoms often reach high biomass and productivity, especially in oligo-mesotrophic lakes (Gruendling, 1971;Cyr, 1998) and can contribute substantially to whole-lake primary production (e.g. Wetzel, 1964;Liboriussen & Jeppesen, 2003;Poulı´cˇkova´et al, 2008;Whalen et al, 2008).…”