The proverbial blue colour of the Mediterranean reflects some of the most extreme oligotrophic waters in the world. Sea-surface Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satelhte data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing oligotrophy eastward, apparent even from space. Integrated over depth, however, the east and west Mediterranean show similar amounts of phytoplankton and bacterial biomass. By contrast, primary production and bacterial production are 2 to 3 times lower in the eastern Mediterranean than in the west. However, the relationship between bacterial production and primary production in the east and west are significantly different. While bacterial production is hrectly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This suggests that the bacteria in the west are relatively decoupled from local contemporaneous primary production. In contrast, the gradient of close to 1 in the log bacterial production versus log primary production relationship in the east suggests less temporal decoupling and, therefore, less seasonal accumulation of DOC. In addition, the constant proportionahty between bacterial and primary production of 0.22, whlch, if all primary products are respired, gives an estimated geometric mean bacteria growth efficiency of 22% (95% confidence limits of 17 and 29%) for data in the eastern Mediterranean. Our data suggest that the degree of bacteria-phytoplankton coupling has an important effect on apparent trends between bacterial and phytoplankton production in high frequency data. The combination of low primary production and bacterial dominance of secondary production in the east is also of significance as it could account for the low fisheries production, the low vertical flux of material and low biomass of benthic organisms in the region.
The sediment-stabilizing effect of benthic diatoms was investigated in a laboratory setting. Axenic cultures of the benthic diatoms Nitzschia cf. brevissima and Cylindrotheca closterium were inoculated in Petri dishes containing sand and incubated under axenic conditions. By ensuring aseptic routines throughout the experiments, interference from other organisms occurring with diatoms in natural photothrophic biofilms was avoided. This allowed the examination of the role of benthic diatoms in sediment stabilization. Increases in the critical erosion shear stress of the sediment were observed in the presence of both diatom taxa relative to sterile sediment. However, N. cf. brevissima was more effective than C. closterium. Values of critical shear stress in the experimental system were in the same range as those observed in natural biofilms, which indicates that diatoms are important agents for biogenic stabilization. Extracellular carbohydrate contents in the microcosms were similar for both diatom species. However, in the presence of N cf. brevissima, extracellular carbohydrate correlated significantly to critical shear stress, explaining up to 80% of the variation, whereas this was not the case for C. closterium. Therefore, it was concluded that the quantity of extracellular polymeric substances (EPS) alone did not explain the biogenic stabilization. Observed adsorption of EPS to sediment particles depended on the relative amount of uronic acids in the exopolymers. Using fluorescently labeled lectins, confocal laser scanning microscopy showed that EPS secretion by N. cf. brevissima resulted in ordered three-dimensional matrix structures. It is suggested that the structuring of EPS plays an prominent role in the process of biostabilization, and that diatoms such as N. cf. brevissima are actively involved in producing the structure of EPS, whereas others such as C. closterium do not do so to the same extent.
Using biological, chemical and physical data, evidence of biologically mediated cohesive sediment dynamics is presented for an intertidal mudflat in the Humber Estuary, UK. The data suggest that in excess of 98% of sediment fluxes to the sediment bed on the mudflat are subject to resuspension, forming a bedload that is important in generating the shear stress required to erode the sediment bed. Benthic micro-algae photosynthesize and are grazed by macro-heterotrophs within this 300-3000 #m deep dynamic surface layer. The data suggest that the algal growth within this layer is limited by nitrogen fluxes, resulting in 'excess' carbon fixation by photosynthesis. A significant positive relationship is apparent between 'excess' carbon fixation rates and the sediment critical erosion stress. This indicates that nitrogen limitation and sediment stability were directly proportional to each other on the mudflat. Grazing of the algae by Macoma balthica (a burrowing bivalve) profoundly affects the carbon and nitrogen cycles in the dynamic surface layer, and hence also 'excess' carbon fixation. We propose that a highly interdependent community of benthic algae, bacteria and macro-heterotrophs, acts to regulate sediment dynamics via small-scale nitrogen cycling and the algal exudation of 'excess' carbon as sediment-binding polymers.
A one-dimensional model is presented which combines simplified biological, chemical and physical dynamics across the sediment-water interface of intertidal mudflat sediments. Interactions between ecology, biogeochemistry and sediment dynamics are simulated during iteration through a short-term sequence of light and tides. The model shows that, under 'normal' intertidal conditions, a biolayer composed of unicellular algae can develop at the sediment-water interface. Rapid algal growth generates regularly recurring nitrogen limited conditions in the biolayer. In the model, the algal carbohydrate polymers produced and exuded during nitrogen limitation profoundly influence model sediment dynamics by adjusting sediment cohesion. Stable, but dynamic, sediment surfaces result in the model from a combination of carbohydrate exuding algae, bacterial remineralization of exuded carbohydrate and grazing of the algae by bioturbating macroheterotrophs. Model predictions for sites on an intertidal mudflat in the Humber Estuary, UK show good agreement with the field observations for a range of different physical, chemical and biological variables. The results imply that sediment accretion may be more directly coupled to nitrogen supply than to sediment supply. The results are discussed in terms of an algal growth advantage to carbohydrate exudation.
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