Abstract
The sea surface microlayer (SML) is the boundary layer between the ocean and the atmosphere and plays a unique role in marine biogeochemistry. Phytoplankton communities in this uppermost surface layer are exposed to extreme ultraviolet (UV) radiation and potentially high nutrient supplies. In order to understand the response of SML communities to such contrasting conditions, we conducted experiments at three different sites, the North Sea (open ocean) and two sites, outer and middle fjord, in the Sognefjord, Norway, with differing physical and chemical parameters. We manipulated light, nitrogen (N) and phosphorus (P) supply to natural communities collected from the SML and compared their response to that of the underlying water (ULW) communities at 1-m depth. Phytoplankton communities in both SML and ULW responded significantly to N addition, suggesting the upper 1-m surface phytoplankton communities were N-limited. While phytoplankton growth rates were higher with high N and high light supply, biomass yield was higher under low light conditions and with a combined N and P supply. Furthermore, biomass yield was generally higher in the ULW communities compared to SML communities. Nutrient and light effects on phytoplankton growth rates, particulate organic carbon (POC) and stoichiometry varied with geographical location. Phytoplankton growth rates in both SML and ULW at the open ocean station, the site with highest salinity, did not respond to light changes, whereas the communities in the middle fjord, characterized by high turbidity and low salinity, did experience light limitation. This work on the upper surface phytoplankton communities provides new insights into possible effects of coastal darkening and increases understanding of oceanic biogeochemical cycling.
Highlights
The sea-surface microlayer (SML), defined as the diffusive boundary layer between the ocean and atmosphere (Liss and Duce 2005), covers the ocean ubiquitiously on a global scale (Wurl et al 2011)
The lower salinity and high turbidity in middle fjord station indicated large freshwater inputs. Samples from both layers, SML and underlying water (ULW), from the open ocean and middle fjord stations were collected during lowto-moderate wind regimes, with average wind speeds of 3.4 ± 0.8 m s–1 and 3.6 ± 1.1 m s–1, respectively
We conclude that the response to light and nutrients by the phytoplankton communities is strongly dependent on geographical locations characterized by different hydrographic conditions
Summary
The sea-surface microlayer (SML), defined as the diffusive boundary layer between the ocean and atmosphere (Liss and Duce 2005), covers the ocean ubiquitiously on a global scale (Wurl et al 2011). Surfactants in the SML (Wurl et al 2011), chromophoric dissolved organic matter (CDOM) (Galgani and Engel 2016) and bacterioneuston communities (Stolle et al 2011) have been reported to be enriched up to 400% compared to the underlying water (ULW) at 1-m depth. Nutrients such as nitrogen (N) and phosphorus (P) have been shown to be enriched in the SML due to both wet and dry atmospheric deposition and may be linked to higher surface. Enrichment or depletion of phytoplankton in the SML has been shown to vary both spatially and temporarily, depending on weather conditions, intensities of radiation and vertical stratification (Cullen et al 1989)
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