Enhanced chlorophyll-a concentration in the wake of Sable Island, eastern Canada, revealed by two decades of satellite observations: a response to grey seal population dynamics?
Authors: Devred Emmanuel , Hilborn Andrea , den Heyer Cornelia Elizabeth 
Published: November 2021

Abstract. Elevated surface chlorophyll-a (chl-a) concentration ([chl-a]), an index of phytoplankton biomass, has been previously observed and documented by remote
sensing in the waters to the southwest of Sable Island (SI) on the Scotian Shelf in eastern Canada. Here, we present an analysis of this phenomenon
using a 21-year time series of satellite-derived [chl-a], paired with information on the particle backscattering coefficient at 443 nm
(bbp(443), a proxy for particle suspension) and the detritus/gelbstoff absorption coefficient at 443 nm (adg(443), a
proxy to differentiate water masses and presence of dissolved organic matter) in an attempt to explain some possible mechanisms that lead to the
increase in surface biomass in the surroundings of SI. We compared the seasonal cycle, 8 d climatology and seasonal trends of surface waters
near SI to two control regions located both upstream and downstream of the island, away from terrigenous inputs. Application of the self-organising
map (SOM) approach to the time series of satellite-derived [chl-a] over the Scotian Shelf revealed the annual spatio-temporal patterns around SI
and, in particular, persistently high phytoplankton biomass during winter and spring in the leeward side of SI, a phenomenon that was not observed
in the control boxes. In the vicinity of SI, a significant increase in [chl-a] and adg(443) during the winter months occurred at a rate
twice that of the ones observed in the control boxes, while no significant trends were found for the other seasons. In addition to the increase in
[chl-a] and adg(443) within the plume southwest of SI, the surface area of the plume itself expanded by a factor of 5 over the last
21 years. While the island mass effect (IME) explained the enhanced biomass around SI, we hypothesised that the large increase in [chl-a] over the
last 21 years was partly due to an injection of nutrients by the island's grey seal colony, which has increased by 200 % during the same
period. This contribution of nutrients from seals may sustain high phytoplankton biomass at a time of year when it is usually low following the fall
bloom. A conceptual model was developed to estimate the standing stock of chl-a that can be sustained by the release of nitrogen (N) by
seals. Comparison between satellite observations and model simulations showed a good temporal agreement between the increased abundance of seal on SI
during the breeding season and the phytoplankton biomass increase during the winter. We found that about 20 % of chl-a standing stock
increase over the last 21 years could be due to seal N fertilisation, the remaining being explained by climate forcing and oceanographic
processes. Although without in situ measurements for ground truthing, the satellite data analysis provided evidence of the impact of marine
mammals on lower trophic levels through a fertilisation mechanism that is coupled with the IME with potential implications for conservation and
fisheries.