d15N of trap WR2

Temporal changes in d15N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in d15N (to as low as 2.5 per mil) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low d15N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High d15N values (up to 13.1 per mil) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower d15N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with d15N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on d15N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on d15N and possible variability in preservation of the isotope signal between periods of high and low particle flux.

Investigation of time series stations along a productivity gradient north of the Canary Islands

Three stations along a productivity gradient north of the Canary Islands were investigated for surface-water properties, particle flux, and composition (biogenic and lithogenic components, and stable nitrogen isotope composition, delta15N) and export production. Investigation sites along the east-west transect off the NW African upwelling margin included the European Station for Time-Series in the Ocean, Canary Islands (ESTOC), one location contiguous to the NW African upwelling zone in the Eastern Boundary Current (EBC) and one station north of the island La Palma (LP). The seasonality of surface-water properties along the transect was mainly influenced by the winter cooling and simultaneous phytoplankton maximum and, in addition at EBC, by nearby upwelling. Accordingly, particle flux and composition along the transect were closely linked to the winter bloom sedimentation and upwelling related enhanced plankton biomass stemming from the primary upwelling and the Cape Yubi filament at EBC. During all seasons, particle flux was highest at EBC and had the highest contribution of biogenic opal and lithogenic components, and the lowest delta15N compared to the offshore stations. But contrary to what would be expected from the productivity gradient, particle flux did not decrease from ESTOC to LP. Below the upper several hundred meters, particle flux was enhanced by additional particle input along the entire transect, manifested by an increase of flux with depth and lower delta15N values. We offer a scenario in which intermediate nepheloid layers originating from the primary upwelling as well as particle dispersion from upwelling filaments, mainly the Cape Ghir filament, impact on the trap stations as far as 700 km into the open ocean. This study contributes to our understanding of the poorly resolved biogeochemical transition between the productive shelf and subtropical gyre provinces.

Planktonic Foraminifera shell calcification intensity from sediment trap L1/K276 from the Azores Front (2002/2003)

Using shells collected from a sediment trap series in the Madeira Basin, we investigate the effects of seasonal variation of temperature, productivity, and optimum growth conditions on calcification in three species of planktonic Foraminifera. The series covers an entire seasonal cycle and reflects conditions at the edge of the distribution of the studied species, manifesting more suitable growth conditions during different parts of the year. The seasonal variation in seawater carbonate saturation at the studied site is negligible compared to other oceanic regions, allowing us to assess the effect of parameters other than carbonate saturation. Shell calcification is quantified using weight and size of individual shells. The size-weight scaling within each species is robust against changes in environmental parameters, but differs among species. An analysis of the variation in calcification intensity (size-normalized weight) reveals species-specific response patterns. In Globigerinoides ruber (white) and Globigerinoides elongatus, calcification intensity is correlated with temperature (positive) and productivity (negative), whilst in Globigerina bulloides no environmental forcing is observed. The size-weight scaling, calcification intensity, and response of calcification intensity to environmental change differed between G. ruber (white) and G. elongatus, implying that patterns extracted from pooled analyses of these species may reflect their changing proportions in the samples. Using shell flux as a measure of optimum growth conditions, we observe significant positive correlation with calcification intensity in G. elongatus, but negative correlation in G. bulloides. The lack of a consistent response of calcification intensity to optimum growth conditions is mirrored by the results of shell size analyses. We conclude that calcification intensity in planktonic Foraminifera is affected by factors other than carbonate saturation. These factors include temperature, productivity, and optimum growth conditions, but the strength and sign of the relationships differ among species, potentially complicating interpretations of calcification data from the fossil record.