| | | 18.8 Mechanisms |
Degobbis et al. [124] have argued that aperiodic discharge of the Po during the early summer delivers nutrients for rapid uptake and assimilation by the late spring diatom community, leading to phosphorus limitation This, in turn, favors the production of high molecular weight exocellular and intracellular saccharides fibrils and through death and decomposition, cell wall debris.
In the Gulf of Trieste, Faganeli and co-authors [151] provided a chronology for mucilage formation, aggregation, and sedimentation in response to discharge of the Isonzo River, indicating that this phenomenon might occur throughout the NA and its adjoining bays. N, P, and silicon inputs in the spring stimulated phytoplankton production, resulting in increases in particulate protein and carbohydrate. Depletion of nutrients led to decreases in protein while particulate carbohydrate concentrations remained unchanged. Marine snow resulted, followed by macroaggregates. After six weeks, the aggregates were most abundant at 10 m with sedimented macroaggregates typified by the same Δ 13C signal, i.e., that of structural heteropolysaccharides, as the initial spring bloom. Later, mucilage was found in superficial bottom sediments.
Accepting the role of P-limited diatom growth as the initial source of polysaccharides, there are three plausible mechanisms for subsequent development of mass mucilage accumulations in the NA. Differences arise on the fate of diatom production and its role in pelagic carbon and nutrient cycling. The first hypothesis favors lysis of diatom populations and accumulation of cell wall and intracellular polysaccharide materials. This lysis hypothesis has only just been proposed and requires basin-wide research on lytic agent distributions and activities. Two other hypotheses are slightly more plausible due to common foundations on characteristic regional water quality and circulation. In one, advocated by Herndl and associates [229], the diatoms become P-limited and produce large amounts of polysaccharides. Ambient bacteria initially hydrolyze the nitrogenous substrates and α-glucosidic polysaccharides, leading to increasing C/N and a more refractory, gel-like aggregate with high proportions of β-glucosidic saccharides in accumulating matrices. P-limited bacterioplankton produce only small amounts of exocellular enzymes capable of hydrolyzing these compounds as well as large molecular weight compounds, including refractory capsular material, further stabilizing the mucilage. The mucilage matrix sinks to the pycnocline, scavenging plankton and other seston. This scenario also has room for some of the lysis argument in that, as aggregates age, cells lyse, release saccharides into the matrix and extend fibril/stringer-like matrices.
Mucilage accumulating at the pycnocline can rise through the water column as a function of photosynthetic activity. This ascent and nocturnal descent favor continued entrapment of plankton and seston from the water column, leading to enrichment of pico-microplankton in the aggregates. Depending on bubble-induced buoyancy versus net density, aggregates can be trapped at the sea surface to yield a photo-oxidized scum or sink to the pycnocline where aggregations aperiodically cascade to the bottom, leading to bottom water anoxia and mass benthic mortalities.
The third hypothesis, principally advocated by Azam and co-authors [27] , suggests that the persistent phytoplankton blooms in the Northern Adriatic Sea are sustained by efficient P remineralization. Bacteria remineralize P in preference to carbon because of high phosphatase activity relative to glucosidase activity (40-53 fold higher activities), yielding organic matter with high C/P, including slowly degrading polysaccharides. P is remineralized from the saccharide accumulations, perpetuating high productivity and additional high C/P polysaccharides. As noted by Herndl and co-workers [229], bacterial capsular material, polysaccharides and mucopolysaccharides, further stabilize the saccharide matrices. Accumulated saccharides rise to the surface not through photosynthetic activity but through bacterial-mediated denitrification and accumulation of N2 in the mats.
The most recent findings [158] indicate that an uncoupling between primary production and bacterial carbon demand can be envisage as one of the most important factor to increasing dissolved organic carbon availability which eventually aggregates as mucilage.
| | | 18.8 Mechanisms |