Abstract
Abstract. Primary production on the coast and in Greenland fjords sustains important local and sustenance fisheries. However, unprecedented melting of the Greenland Ice Sheet (GrIS) is impacting the coastal ocean, and its effects on fjord ecology remain understudied. It has been suggested that as glaciers retreat, primary production regimes may be altered, rendering fjords less productive. Here we investigate patterns of primary productivity in a northeast Greenland fjord (Young Sound, 74∘ N), which receives run-off from the GrIS via land-terminating glaciers. We measured size fractioned primary production during the ice- free season along a spatial gradient of meltwater influence. We found that, apart from a brief under-ice bloom during summer, primary production remains low (between 50 and 200 mg C m−2 d−1) but steady throughout the ice-free season, even into the fall. Low productivity is due to freshwater run-off from land-terminating glaciers causing low light availability and strong vertical stratification limiting nutrient availability. The former is caused by turbid river inputs in the summer restricting phytoplankton biomass to the surface and away from the nitracline. In the outer fjord where turbidity plays less of a role in light limitation, phytoplankton biomass moves higher in the water column in the fall due to the short day length as the sun angle decreases. Despite this, plankton communities in this study were shown to be well adapted to low-light conditions, as evidenced by the low values of saturating irradiance for primary production (5.8–67 µmol photons m−2 s−1). With its low but consistent production across the growing season, Young Sound offers an alternative picture to other more productive fjords which have highly productive spring and late summer blooms and limited fall production. However, patterns of primary productivity observed in Young Sound are not only due to the influence from land-terminating glaciers but are also consequences of the nutrient-depleted coastal boundary currents and the shallow entrance sill, features which should also be considered when generalizing about how primary production will be affected by glacier retreat in the future.
Highlights
The coastal marine coastal ecosystems around Greenland are currently experiencing rapid changes due to climate warming
Patterns of primary productivity observed in Young Sound are due to the influence from land-terminating glaciers but are consequences of the nutrient-depleted coastal boundary currents and the shallow entrance sill, features which should be considered when generalizing about how primary production will be affected by glacier retreat in the future
Primary production rates in this data set the fall within the range of values measured previously in Young Sound by Rysgaard et al (1999) (this data set: 206.8– 10.8 mg C m−2 d−1 (Figs. 5b and 6a; Table 1); Rysgaard et al (1999): 277.9–4.2 mg C m−2 d−1) with the exception of the high rates of primary production measured under the ice at Station 3 on 11 July
Summary
The coastal marine coastal ecosystems around Greenland are currently experiencing rapid changes due to climate warming. Holding et al.: Seasonal and spatial patterns of primary production in a high-latitude fjord discharge into the surrounding coastal region has increased by almost 50 % over just the last 2 decades (Bamber et al, 2012). This reported freshening of fjords and coastal waters around Greenland (Böning et al, 2016; Sejr et al, 2017) has major consequences for the marine ecosystem as well as for the inshore fisheries (Meire et al, 2017). The magnitude and direction of these effects on the different fjord ecosystems around Greenland are still largely unclear
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