Nutrient regeneration in the deep water of sea lochs.

Grantham, Brian Edwin (1987). Nutrient regeneration in the deep water of sea lochs. MPhil thesis The Open University.



Changes in the nutrient concentrations in a Scottish sea Loch, Loch Etive, were studied over a period of 2 years. During most of this period the water below 65 m in a basin of depth 150 m was essentially isolated through freshwater-induced density stratification. Renewals of the deep water occurred only during infrequent prolonged dry spells when the near-surface density rose sufficiently to allow density current inflow to penetrate to the bottom.

Nutrient concentrations in the stagnant water rose from low levels at the start of isolation to values of between 1 and 3 times the normal winter maxima for the adjacent coastal area, after two years. Dissolved oxygen showed a corresponding fall from 100% saturation to 20-25% saturation over the same period. Anoxic conditions were not observed. The greatest changes occurred in the first 6 months of isolation, with much smaller changes thereafter; equilibrium was eventually reached between the rate of supply (of nutrients) or consumption (of oxygen) and diffusional flows.

Terrigenous material dominated the organic input, but was more refractory than material of marine origin. Primary production provided more than adequate organic matter to account for the observed regeneration. Most of the nutrient increase in the water column was accounted for by regeneration in the sediments. Flux rates per unit area of sediment agreed with published figures for coastal and estuarine conditions. Stoichiometric ratios of nutrient and dissolved oxygen changes suggested a deficit in nitrate and a surplus of silicate. Nitrate was probably lost through denitrification in the sediments. Marine organic matter seemed to be the major source of material for regenerated nutrients, although the excess silicate
indicated some possible regeneration from terrigenous material.

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