Impact of glacial meltwater on spatiotemporal distribution of copepods and their grazing impact in Young Sound NE, Greenland

Abstract

The melting of sea ice, increased terrestrial runoff and meltwater from the Greenland Ice Sheet contribute to increased freshening and stratification of Arctic seas. However, the impacts on ecosystem structure and carbon cycling are still poorly quantified. We studied copepod abundance, grazing, and carbon turnover in Young Sound, Northeast Greenland, during the ice-free period from mid-July to early October 2014 at four stations along the freshwater gradient from the inner fjord to the Greenland Sea. Freshwater enters the fjord as surface runoff from land-terminating glaciers and forms a surface layer of high turbidity and low salinity limiting light availability and nutrient replenishment. This resulted in the inner station consistently having lower biomass of chlorophyll a, (average 21 mg m 22) compared to the outer station (47 mg m 22) and phytoplankton cells smaller than 10 lm constituted 89% at the inner station compared to 59% at the outer station. Copepod grazing rates were low on these small cells, which resulted in low copepods community grazing at the freshwater impacted site in the inner fjord. Daily grazing impact by copepods was about half of the daily primary production during July and August in the outer region of the fjord, but grazing impact was always less than 10% of the daily primary production in the inner fjord and in the late part of the open water season at all stations. Our study shows that meltwater driven stratification results in dominance of small phytoplankton cells, which directly impacts secondary producers and reduces transfer to higher trophic levels.

Biological transformation of Arctic dissolved organic matter in a NE Greenland fjord

“…3 and 4). In the outer part of the fjord, diatoms, mainly Chaetoceros sp., and autotrophic dinoflagellates dominated, while picophytoplankton dominated the inner fjord (Middelbo et al ). Large and small heterotrophic HNFs also had different distribution patterns with large HNF being most abundant in the inner part of the fjord in autumn (max.…”

“…Water for each treatment was gently transferred into triplicate 5‐liter transparent foldable politainers (RPC Promens®) by staggered filling using silicone tubing. To mirror the seasonal change in mesozooplankton biomass in the fjord (Middelbo et al ), Exp. 2 and 3 included additional treatments where mesozooplankton were added to < 90 μ m filtered water.…”

“…1, there were relatively few copepods present in the fjord (a biomass of < 0.5 mg C m −3 ), therefore a treatment with copepod addition was not included. After the ice broke up copepod abundance increased up to 5 mg C m −3 (Middelbo et al ) and a “copepod treatment” was included both in Exp. 2 and 3.…”

“…Further, the silty glacial runoff drastically reduces the euphotic depth. The effects of the glacial runoff on the distribution of micro‐ and mesozooplankton (Arendt et al ; Middelbo et al ), marine ecosystem productivity (Meire et al ), colored DOM absorption (Murray et al ), and the carbon availability for bacteria (Paulsen et al ) have previously been described for Young Sound. Small phytoplankton (< 2 μ m) dominate the murky inner fjord and contribute on average 89% of chlorophyll a (Chl a ) compared to 59% at the mouth of the fjord.…”

“…Small phytoplankton (< 2 μ m) dominate the murky inner fjord and contribute on average 89% of chlorophyll a (Chl a ) compared to 59% at the mouth of the fjord. At the same time, copepod grazing is considerably less in the inner fjord (Middelbo et al ), while protist grazing is enhanced (Arendt et al ). Copepods and protists generally dominate the grazer biomass in Young Sound, although the pelagic gastropod Limacina helicina is abundant particularly in autumn (Middelbo et al ).…”