•Seasonality and cross shelf distribution of DOM and its optical properties were examined in the Celtic Sea.•Strong cross shelf gradients in humic DOM components with significant negative ...correlations with salinity and DOC.•Variability in labile DOM protein components linked to biological productivity.
The Celtic Sea is a productive temperate sea located on the Northwest European Shelf. It is an important pathway for the delivery of land-derived material to the North Atlantic Ocean, including dissolved organic matter (DOM). The aim of this study was to determine the seasonal and spatial variability in the magnitude, source and composition of DOM at three sites representing on shelf, central shelf and shelf edge regions in the Celtic Sea, using observations collected during the UK Shelf Sea Biogeochemistry (SSB) research programme (November 2014 – August 2015). The concentration of dissolved organic carbon (DOC) alongside DOM absorbance and fluorescence indices were measured and fluorescence Excitation and Emission Matrices (EEMs) combined with Parallel Factor Analysis (PARAFAC) were used to assess DOM composition and lability. The PARAFAC model identified four unique fluorescent components for autumn (November 2014), winter (March 2015), spring (April 2015) and summer (July 2015) consisting of two humic-like components attributed to terrestrial (C1) and marine sources (C2), and two protein components identified as tyrosine-like (C3) and tryptophan-like (C4) attributed to in situ production. DOC varied seasonally and there were strong cross shelf trends. The protein components (C3 and C4) exhibited large seasonal and within season variability particularly during productive periods. In contrast, there were persistent cross shelf gradients in the CDOM absorption coefficient at 305 nm (a305), the UV specific absorbance at 280 nm (SUVA280), the humification index (HIX), and the humic-like fluorescent components (C1 and C2), which were higher in the on shelf region and decreased towards the shelf edge. The humic-like components and the slope ratio (SR) were significantly correlated with salinity throughout all seasons, indicating a strong influence of terrestrially-derived organic matter in the Celtic Sea, with potentially up to 35% of DOC in the central shelf during winter originating from terrestrial inputs. Results from this study illustrate the importance of monitoring DOM quantitatively and qualitatively for a better understanding of the supply, production, cycling and export of this dynamic organic carbon pool in shelf seas.
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Zooplankton on continental shelves represent an important intermediary in the transfer of energy and matter from phytoplankton to the wider ecosystem. Their taxonomic composition and ...trophic interactions with phytoplankton vary in space and time, and interpreting the implications of this constantly evolving landscape remains a major challenge. Here we combine plankton taxonomic data with the analysis of biovolume spectra and stable isotopes to provide insights into the trophic interactions that occur in a shelf sea ecosystem (Celtic Sea) across the spring-summer-autumn transition. Biovolume spectra captured the seasonal development of the zooplankton community well, both in terms of total biomass and trophic positioning, and matched trophic positions estimated by stable isotope analysis. In early April, large microplankton (63–200 µm) occupied higher trophic positions than mesozooplankton (>200 µm), likely reflecting the predominance of nanoplankton (2–20 µm) that were not readily available to mesozooplankton grazers. Biomass and number of trophic levels increased during the spring bloom as elevated primary production allowed for a higher abundance of predatory species. During July, the plankton assemblage occupied relatively high trophic positions, indicating important links to the microbial loop and the recycling of organic matter. The strong correlation between biomass and community trophic level across the study suggests that the Celtic Sea is a relatively enclosed and predominantly energy-limited ecosystem. The progression of the zooplankton biomass and community structure within the central shelf region was different to that at the shelf-break, potentially reflecting increased predatory control of copepods by macrozooplankton and pelagic fishes at the shelf break. We suggest that the combination of size spectra and stable isotope techniques are highly complementary and useful for interpreting the seasonal progression of trophic interactions in the plankton.
The mesopelagic zooplankton community plays an important role in the cycling and sequestration of carbon via the biological pump. However, little is known about the physiology and ecology of key taxa ...found within this region, hindering our understanding of their influence on the pathways of energy and organic matter cycling. We sampled the eight most abundant zooplankton (Calanoides acutus, Rhincalanus gigas, Paraeuchaeta spp., Chaetognatha, Euphausia triacantha, Thysanoessa spp., Themisto gaudichaudii and Salpa thompsoni) from within the mesopelagic zone in the Scotia Sea during a sinking diatom bloom and investigated their physiological ecology using lipid biomarkers and stable isotopic signatures of nitrogen. Data suggest that the large calanoid copepods, C. acutus and R. gigas, were in, or emerging from, a period of metabolic inactivity during the study period (November 15th – December 15th, 2017). Abundant, but decreasing lipid reserves in the predominantly herbivorous calanoid copepods, suggest these animals may have been metabolising previously stored lipids at the time of sampling, rather than deriving energy solely from the diatom bloom. This highlights the importance of understanding the timing of diapause of overwintering species as their feeding is likely to have an impact on the turnover of particulate organic matter (POM) in the upper mesopelagic. The δ15N signatures of POM became enriched with increasing depth, whereas all species of zooplankton except T. gaudichaudii did not. This suggests that animals were feeding on fresher, surface-derived POM, rather than reworked particles at depth, likely influencing the quantity and quality of organic matter leaving the upper mesopelagic. Our study highlights the complexity of mesopelagic food webs and suggests that the application of broad trophic functional types may lead to an incorrect understanding of ecosystem dynamics.
•Calanoides acutus and Rhincalanus gigas use stored lipids for energy not ingestion.•Actively feeding zooplankton consumed fresh POM, not reworked particles at depth.•Decoupling between zooplankton feeding dynamics and ambient POM in the upper 500m.
Shelf seas are highly productive and economically important regions of our ocean, and therefore play a significant role in the global ocean carbon cycle. While accounting for only 8% of the ocean ...surface area they support between 15 and 30% of global ocean primary productivity (Wollast 1998) and are responsible for up to 40% of global ocean carbon sequestration (Muller‐Karger et al. 2005). Their productivity is thought to be sustained through dynamic physical mixing processes combined with the supply of both terrestrial and oceanic nutrients and internal regeneration of nutrients. However, the relative importance of these different nutrient pools is currently not clear. Nitrogen is often a key limiting nutrient for biological production within the open ocean and shelf seas and therefore has an important role in ocean biogeochemistry influencing the marine carbon and phosphorus cycles. The processes within the nitrogen cycle are complex. This study aims to identify the sources of nitrogen to the NW European shelf and to understand how nitrogen is cycled on the shelf.In this thesis, the nitrogen stable isotope composition the δ15N and stable oxygen isotope (δ18O) composition of dissolved nitrate has been used to quantitatively assess the relative magnitude and importance of the oceanic source of nitrate versus on shelf regeneration of nitrate in sustaining the on shelf nitrate pool that supports the high primary productivity in temperate shelf seas.In the Celtic Sea there was greater in-situ remineralisation of nitrate across the inner shelf during the summer (≤ 30 %) and autumn (≤ 60 %) in 2015 indicated by the decoupling of the δ15NNO3 and δ18ONO3 in bottom water with the introduction of isotopically light δ18ONO3. The influence of riverine water was also greater at the inner shelf stations as evidenced by a mixing of riverine and oceanic isotope end-members. In contrast, the proportion of nitrate remineralised in-situ across the outer shelf region of the Celtic Sea during the summer and autumn period was lower (0 to 10 %), with an off shelf supply of nitrate being more important than previously thought. Across the Hebrides shelf a significant proportion (up to 94 %) of the nitrate was remineralised in-situ at the inner and middle shelf stations during autumn 2014. The shelf edge was influenced by the off shelf supply of nitrate and the inner shelf regions were influenced by the Scottish Coastal Current and greater in-situ remineralisation.The results of this study have provided the first assessment of the regeneration of nitrogen in the North West European shelf seas using the stable isotopes of nitrate. This has improved the knowledge of nitrogen cycling on the shelf and identified the magnitude of the sources of nitrogen to the shelf over an annual cycle. These results have significant implications for the efficiency of shelf sea carbon pump and the method of subsequent carbon export from shelf seas.
Shelf seas are highly productive and economically important regions of our ocean, and therefore play a significant role in the global ocean carbon cycle. While accounting for only 8% of the ocean ...surface area they support between 15 and 30% of global ocean primary productivity (Wollast 1998) and are responsible for up to 40% of global ocean carbon sequestration (Muller-Karger et al. 2005). Their productivity is thought to be sustained through dynamic physical mixing processes combined with the supply of both terrestrial and oceanic nutrients and internal regeneration of nutrients. However, the relative importance of these different nutrient pools is currently not clear. Nitrogen is often a key limiting nutrient for biological production within the open ocean and shelf seas and therefore has an important role in ocean biogeochemistry influencing the marine carbon and phosphorus cycles. The processes within the nitrogen cycle are complex. This study aims to identify the sources of nitrogen to the NW European shelf and to understand how nitrogen is cycled on the shelf. In this thesis, the nitrogen stable isotope composition the d15N and stable oxygen isotope (d18O) composition of dissolved nitrate has been used to quantitatively assess the relative magnitude and importance of the oceanic source of nitrate versus on shelf regeneration of nitrate in sustaining the on shelf nitrate pool that supports the high primary productivity in temperate shelf seas. In the Celtic Sea there was greater in-situ remineralisation of nitrate across the inner shelf during the summer (= 30 %) and autumn (= 60 %) in 2015 indicated by the decoupling of the d15NNO3 and d18ONO3 in bottom water with the introduction of isotopically light d18ONO3. The influence of riverine water was also greater at the inner shelf stations as evidenced by a mixing of riverine and oceanic isotope end-members. In contrast, the proportion of nitrate remineralised in-situ across the outer shelf region of the Celtic Sea during the summer and autumn period was lower (0 to 10 %), with an off shelf supply of nitrate being more important than previously thought. Across the Hebrides shelf a significant proportion (up to 94 %) of the nitrate was remineralised in-situ at the inner and middle shelf stations during autumn 2014. The shelf edge was influenced by the off shelf supply of nitrate and the inner shelf regions were influenced by the Scottish Coastal Current and greater in-situ remineralisation. The results of this study have provided the first assessment of the regeneration of nitrogen in the North West European shelf seas using the stable isotopes of nitrate. This has improved the knowledge of nitrogen cycling on the shelf and identified the magnitude of the sources of nitrogen to the shelf over an annual cycle. These results have significant implications for the efficiency of shelf sea carbon pump and the method of subsequent carbon export from shelf seas.
Shelf seas are highly productive and economically important regions of our ocean, and therefore play a significant role in the global ocean carbon cycle. While accounting for only 8% of the ocean ...surface area they support between 15 and 30% of global ocean primary productivity (Wollast 1998) and are responsible for up to 40% of global ocean carbon sequestration (Muller-Karger et al. 2005). Their productivity is thought to be sustained through dynamic physical mixing processes combined with the supply of both terrestrial and oceanic nutrients and internal regeneration of nutrients. However, the relative importance of these different nutrient pools is currently not clear. Nitrogen is often a key limiting nutrient for biological production within the open ocean and shelf seas and therefore has an important role in ocean biogeochemistry influencing the marine carbon and phosphorus cycles. The processes within the nitrogen cycle are complex. This study aims to identify the sources of nitrogen to the NW European shelf and to understand how nitrogen is cycled on the shelf. In this thesis, the nitrogen stable isotope composition the d15N and stable oxygen isotope (d18O) composition of dissolved nitrate has been used to quantitatively assess the relative magnitude and importance of the oceanic source of nitrate versus on shelf regeneration of nitrate in sustaining the on shelf nitrate pool that supports the high primary productivity in temperate shelf seas. In the Celtic Sea there was greater in-situ remineralisation of nitrate across the inner shelf during the summer (= 30 %) and autumn (= 60 %) in 2015 indicated by the decoupling of the d15NNO3 and d18ONO3 in bottom water with the introduction of isotopically light d18ONO3. The influence of riverine water was also greater at the inner shelf stations as evidenced by a mixing of riverine and oceanic isotope end-members. In contrast, the proportion of nitrate remineralised in-situ across the outer shelf region of the Celtic Sea during the summer and autumn period was lower (0 to 10 %), with an off shelf supply of nitrate being more important than previously thought. Across the Hebrides shelf a significant proportion (up to 94 %) of the nitrate was remineralised in-situ at the inner and middle shelf stations during autumn 2014. The shelf edge was influenced by the off shelf supply of nitrate and the inner shelf regions were influenced by the Scottish Coastal Current and greater in-situ remineralisation. The results of this study have provided the first assessment of the regeneration of nitrogen in the North West European shelf seas using the stable isotopes of nitrate. This has improved the knowledge of nitrogen cycling on the shelf and identified the magnitude of the sources of nitrogen to the shelf over an annual cycle. These results have significant implications for the efficiency of shelf sea carbon pump and the method of subsequent carbon export from shelf seas.
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