Carbohydrates in the North Sea during spring blooms of <i>Phaeocystis</i>: a specific fingerprint

Regional and temporal variation in the composition of water-soluble carbohydrates from Phaeocystis colonies sampled in the southern North Sea was small during spring 1994, except for a high variability in the contribution of glucose. Glucose is universally present in storage products of microalgae; the relative constancy of the carbohydrate pattern of the other monosaccharides suggests that these are part of the more refractory colony mucus. In all Phaeocystis samples arabinose dominated, followed by xylose (Belgian coast) or galactose and mannose (Dutch coast). Rhamnose, glucuronate and O-methylated sugars were present in lower amounts. The latter, always present in samples containing Phaeocystis, may be typical for North Sea strains. The sugar patterns we report here differ from those presented in the literature concerning Phaeocystis-derived material, and also from the sugar fingerprint in the preceding diatom bloom. The Phaeocystis mucus apparently behaves as particulate matter since it was retained on filters of over 1 um. This characteristic together with its refractory nature, typical of 'transparent exopolymer particles' (TEPs), must have consequences for the heterotrophic microbial community in terms of adherence and substrate availability

Primary production, nutrients, and size spectra of suspended particles in the southern North Sea

The effect of nutrient enrichment from the Rhine on some major characteristics of the phytoplankton ecosystem of Dutch coastal waters was studied with 14C, liquid scintillation and Coulter Counter techniques. The magnitude of primary production in the most eutrophic waters closest to shore and the Rhine mouth was found to be affected by turbidity, rather than by concentration of nutrients only. That the annual production cycle starts later in onshore waters than in the clear offshore water mass is probably also due to turbidity. Primary production of dissolved organic matter was at times up to 20% of the total carbon fixed per square meter. It is suggested that filtrate radio-activity is to a large, though unknown, extent derived from labelled organic compounds of the mucus of colony-forming species like Phaeocystis globosa and Chaetoceros socialis. The nutrient regime and degree of eutrophication, but also other factors like stratification, turbulence, and light conditions seem to control the community structure and species composition of the populations, and therewith the amount of primary productivity. Diatom productivity in the Dutch coastal water mass can be high even in summer, in spite of a near-depletion of silicate already early in spring. This can be so because silicate concentrations may periodically rise, e.g. in stratified water due to the dominance of dinoflagellates in the primary producer populations , or after periods with certain climatic-hydrographic conditions when diatoms may sink out of the euphotic zone

The phytoplankton spring bloom in Dutch coaqtal waters of the North Sea

In the eastern part of the Southern Bight of the North Sea several sub areas could be distinguished, each with a characteristic spring bloom and species succession pattern. Regional differences in spring bloom timing were in accord with theoretical considerations in which (on the assumption of a vertically homogeneous distribuvion of phytoplankton) turbidity, incident radiation and water depth are key factors. The development rate of the spring bloom in Southern Bight waters can therefore be expected to change from year to year due to shifts in the balance between the effect of turbidity on the phytoplankton and the increasing solar radiation in spring. The growing season does not start anywhere in the southern North Sea before the mean light intensivy of the water column has passed 0.03 gcal cm-2min-1. After this the spring bloom development rate is most rapid in either clear or shallow waters (phytoplankton concentrations over 1 mm3/l from February to early March in offshore waters and in the French Flemish Banks area), slowest in turbid Dutch coastal waters (more than 1 mm3/l not before the end of March). Conversion of solar energy into organic matter is least efficient in the most turbid zone closest to the Rhine outflow (more than 1 mm3/l of phytoplankton not before the latter half of April); this, plus the lateness of vernal biomass increase, causes a decreased nutrient consumption in the coastal zone most under the influence of eutrophication. Consequently the effect of nutrient enrichment by the Rhine on primary production in the southern North Sea will be perceptible over an increasingly large area with an increase in turbidity and vice versa. During the first half of the growing season of 1974 (February to July) primary production (of particulate plus dissolved organic matter) in the Dutch coastal zone (salinity less than 32‰ S) was 80 to 90 g C/m2; in offshore waters (32 to 35‰ S) 130 to 150 g C/m2. The diatom spring bloom is controlled by silicate, while the maximum size of the succeeding Phaeocystis crop (April to early May) may still be set by phosphate, in spite of the greatly increased P loading of the eastern part of the Southern Bight in the last decades. Chlorophyll a concentrations at the surface usually correspond with surface primary production for a given incident radiation; but chlorophyll cannot be used as an index of phytoplankton concentration (measured as cell volume). Phytoplankton fluctuations in any one region do not only depend upon rate of primary production or of grazing, but also upon currents that may transport phytoplankton patches through that region

Eutrophication of Dutch coastal waters

The concentrations of phosphorus and nitrogen compounds in the river Rhine have increased by a factor of about 7 since 1932: in recent years the rate of increase for P has been higher than for N. The concentration of reactive silicate, which is low in summer, has remained essentially constant. Because in the Southern Bight of the North Sea the contribution of nutrients from the deep oceanic reservoir is minor, these increases in riverborne nutrients have had a marked influence on nutrient values and nutrient ratios of the whole area, but especially so on the narrow strip of water along the Dutch Coast, which has an average salinity of 30‰. These changes have caused silicon to be the first depleted nutrient element, limiting diatom blooms. It is shown that the spring bloom of Phaeocystis poucheti is occasionally able to consume all phosphorus, leaving some nitrogen. However, on the average the phosphorus and nitrogen left over by diatoms after all reactive silicate has been depleted is not consumed by other phytoplankton species. In the strip of 30‰ salinity water along the coast daily primary production may be up to 3500 mg carbon/m2, but yearly primary production is not very high. The vegetative season lasts only about 6 months, due to the influence of turbidity, and is characterized by a succession of periods with high and low productivity. This instability in the phytoplankton regime sometimes causes very high phytoplankton crops and at other times advection and diffusion of unused nutrient loads from the rivers to other parts of the North Sea

Regional and seasonal differences in light absorption by yellow substance in the Southern Bight of the North Sea

Absorption spectra (250-800 nm) of yellow substance were measured in 1993 and 1994 along five onshore-offshore transects in the Southern Eight of the North Sea in different seasons. All spectra had a common shape, so the amount of yellow substance could be established using one reference wavelength as a proxy variable. The exponential slope parameter at 380 nm (S) of the absorption spectra ranged from -0.016 to -0.023 per nm when regression was based upon the 250-650 nm wavelength range; values were 25% lower when fitting was performed over the more restricted waveband 360-540 nm, partly due to a minor shoulder at 260-275 nm in all spectra. The concentration of yellow substance, expressed as the absorption coefficient at 380 nm (a(380); m(-1)), varied over an order of magnitude, from low values down to 0.17 m(-1) in the English Channel, up to 1.00-1.25 m(-1) near the Rhine and Scheldt outflow, to maxima of up to 1.75 m(-1) in the neritic-estuarine waters at the entrance to the shallow Wadden Sea. This regional distribution pattern was in agreement with the known water-mass circulation and with the location of sources of yellow substance: highest concentrations close to the shore under the influence of freshwater inflow, lower values with increasing distance from the coast, where Atlantic Ocean water is the major water-mass component. No significant seasonal variation in yellow-substance concentration was found anywhere when a correction was applied for salinity. Extrapolation to zero salinity yielded an absorption coefficient at 380 nm of 5.38 m(-1) for the freshwater input at both the Texel and Walcheren transects, but of 3.29 m(-1) at the Noordwijk transect, reflecting differences in yellow-substance concentration between the Rhine, the Meuse, and Lake IJssel, as previously noted in fluorescence measurements. Yellow-substance concentration was not only correlated with salinity but also, albeit much less, with chlorophyll concentrations; the contribution of phytoplankton to the yellow-substance pool was marginal but significant. A relation appeared to exist between yellow-substance absorbance at 380 nm and yellow-substance fluorescence intensity; fluorescence measurements in the southern North Sea can be translated to absorption (more appropriate for ocean colour detection by remote sensing) by equating I mFl unit to an absorption coefficient at 380 nm of 0.056 m(-1), and using the exponential relation a(ys)(lambda) = a(ys)(lambda(ref)) exp(-S(lambda - lambda(ref))) for extrapolation to UV-Vis spectral absorption. (C) 1999 Elsevier Science B.V. All rights reserved

Variation of particulate carbohydrate pools over time and depth in a diatom-dominated plankton community at the Antarctic Polar Front

Carbohydrate dynamics were studied in an algal community dominated by the diatom Fragilariopsis kerguelensis at the Antarctic Polar Front during austral autumn 1999. Water-extractable mono- and polysaccharide concentrations from the particulate fraction were measured at six depths in the upper 100 in at seven stations along a N-S transect (20degreesE, 48-50degreesS) during 2 consecutive days. In addition, field populations were incubated on deck for 18 h at four different light intensities. Polysaccharide concentrations varied between 2.7 and 13.6 mug/l, monosaccharide concentrations between 2.5 and 8.9 mug/l. Near the surface (0-60 in), a diet pattern was observed in the polysaccharide concentration when normalised to the chlorophyll a or the monosaccharide concentration. The deck incubations supported the hypothesis that this pattern resulted from the diurnal accumulation and nocturnal consumption of reserve glucan. More polysaccharides accumulated at high light intensities than at low light intensities, in accordance with the observed decrease in normalised polysaccharide concentration with depth. In addition, the lower concentrations at depth might be explained by consumption: polysaccharides that were accumulated during time spent near the surface were subsequently respired when cells were transported to deeper and dimmer water layers. The variation in carbohydrate pools over-time and depth described here must be considered of ecological relevance to phytoplankton in the Southern Ocean subjected to extended periods of darkness (hours to days) due to vertical mixing and advection