Enhanced chlorophyll-&amp;lt;i&amp;gt;a&amp;lt;/i&amp;gt; concentration in the wake of Sable Island, eastern Canada, revealed by two decades of satellite observations: a response to grey seal population dynamics?

Emmanuel Devred,Cornelia Elizabeth Den Heyer,Andrea Hilborn

doi:10.5194/bg-18-6115-2021

Enhanced chlorophyll-&lt;i&gt;a&lt;/i&gt; concentration in the wake of Sable Island, eastern Canada, revealed by two decades of satellite observations: a response to grey seal population dynamics?

Emmanuel Devred, Cornelia Elizabeth Den Heyer + Show 1 more

Open Access

https://doi.org/10.5194/bg-18-6115-2021

Copy DOI

Abstract

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.

Highlights

Increased phytoplankton production around islands, known as the island mass effect (IME), has been documented in many studies since the development of the concept (Doty and Oguri, 1956)
We have developed a conceptual model to describe the seasonal variation in the number of adult seals hauling out on the island based on the estimates of the number of pups and age 1+ animals for the Scotian Shelf population, of which the Sable Island (SI) breeding colony accounts for more that 95 % of the pup production (Rossi et al, 2021) and the analysis of satellite-telemetry data from more than 100 seals that were tagged on SI (O’Boyle and Sinclair, 2012; Breed et al, 2013)
The backscattering coefficient at 443 nm showed a very different pattern than [chla] and adg(443), with values remaining relatively high in winter and early spring followed by a small increase corresponding to the peak of the spring bloom and a decrease in late spring at the end of the spring bloom (Fig. 2c)

Summary

Introduction

Increased phytoplankton production around islands, known as the island mass effect (IME), has been documented in many studies since the development of the concept (Doty and Oguri, 1956). Phytoplankton biomass is subject to a strong seasonal cycle, with a spring bloom representing a major food input into the ecosystem and a secondary fall bloom triggered by the replenishment of nutrients to the surface-lit layer as a result of physical forcing (Song et al, 2010) While this general progression is well established, the mesoscale patterns of phytoplankton dynamics are more complex given the complicated bathymetry and hydrodynamics of the Scotian Shelf. A feature that remains constant is the moderate to high surface phytoplankton biomass that occurs leeward of SI in comparison to its surroundings as observed by satellite OCRS (Fig. 1) While this has been documented in previous studies (King et al, 2016; Zhai et al, 2011), the temporal and spatial dynamics of this plume off SI have not been examined using a 21-year time series of satellite records. Previous studies have demonstrated the impact of marine mammals on the supply of nutrients by direct release of N in the marine environment (Roman and McCarthy, 2010; Laver et al, 2012; Mccauley et al, 2012; Wing et al, 2014), through turbulent mixing (Kanwisher and Ridgway, 1983) and atmospheric deposition (Theobald et al, 2006)

Objectives

Results

Conclusion