Differential contribution of diatoms and dinoflagellates to phytoplankton biomass in the NE Atlantic Ocean and the North Sea

Sampling by the continuous plankton recorder (CPR) survey over the North Atlantic Ocean and the North Sea has enabled long-term studies of phytoplankton biomass. Analysis of an index of phytoplankton biomass, the phytoplankton colour index (PCI), has previously shown an increase in phytoplankton biomass in the NE Atlantic. In the current study, further investigations were conducted to determine the contribution of diatom and dinoflagellate cell counts to the PCI, their fluctuations over the last 45 yr and their geographical variations in the eastern North Atlantic and the North Sea. An increased contribution of dinoflagellates to the PCI was revealed over the south NE Atlantic and the northern North Sea. In contrast, the contribution of diatoms decreased in the north NE Atlantic and the northern North Sea. No discernible trends were found in the other regions of the North Sea. The relative contributions of diatoms and dinoflagellates to the PCI led to the identification of 3 geographically distinct dynamic regimes in the diatom/dinoflagellate dynamics in the NE Atlantic and the North Sea. Finally, it is stressed that the discrepancy observed in the patterns of PCI and diatom and dinoflagellate cell counts suggests that changes in PCI do not reflect changes in the community structure and that the exclusive use of PCI is not adequate to investigate the long-term trends in the trophic link between phytoplankton and herbivorous zooplankton.

Aquatic Distribution And Heterotrophic Degradation Of Polycyclic Aromatic-Hydrocarbons (PAH) In The Tamar Estuary

Variations in the concentrations and microheterotrophic degradation rates of selected Polycyclic Aromatic Hydrocarbons (PAH) in the water column of the Tamar Estuary were investigated in relation to the major environmental variables. Concentrations of individual PAH varied typically between i and 50 ng l−1 Based on their observed environmental behaviour the PAH appeared divisible into two groupings: (1) low molecular weight PAH incorporating naphthalene, phenanthrene and anthracence and (a) the larger molecular weight homologues (fluoranthene, pyrene, chrysene, benz(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)-pyrene). Group 1 PAH showed a complex distribution throughout the estuary with no significant correlations with either salinity or suspended particulates. Based on their relatively low particle affinity and high water solubilities and vapour pressures, volatilization is proposed as an important process in determining their fate. Microheterotrophic turnover times of naphthalene varied between x and 30 days, and were independent of suspended solids with maximum degradation rates located in the central and urban regions of the Estuary. When compared with the flushing times for the Tamar (3–5 days), it is probable that heterotrophic activity is important in the removal of naphthalene (and possibly the other Group 1 PAH) from the estuarine environment. In contrast Group 2 PAH concentrations exhibited highly significant correlations with suspended particulates. Highest concentrations occurred at the turbidity maximum, with a secondary concentration maximum localized to the industrialized portion of the estuary and associated with anthropogenic inputs. Laboratory degradation studies of benzo(a)pyrene in water samples taken from the estuary showed turnover times for the compound of between 2000 and 9000 days. Degradation rates correlated positively with suspended solids. The high particulate affinity and microbial refractivity of Group 2 PAH indicate sediment burial as the principal tate of these PAH in the Tamar Estuary. Estuarine sediments contained typically 50–1500 ng g−1 dry weight of individual PAH which were comparable to the levels of Group 2 PAH associated with the suspended particulates. Highest concentrations occurred at the riverine end of the estuary resulting from unresolved inputs in the catchment. Subsequent dilution by less polluted marine sediments together with slow degradation results in a seaward trend of decreasing concentrations. However, there is a secondary maximum of PAH superimposed on this trend which is associated with urban Plymouth.