LONG-TERM NUTRIENT ENRICHMENT OF AN ESTUARINE SYSTEM - YTHAN, SCOTLAND (1958-1993)

A comprehensive review is given of past and current annually averaged nutrient concentrations (total oxidised nitrogen [TON], phosphate and silicate) in the River Ythan and its estuary. TON concentrations in th e River Ythan have increased from ca 100-150 mu M in the late 1960s to ca 500-550 mu M in the early 1990s, changes which are also shown in t he estuary. The interpretation of the phosphate data is more complex b ut concentrations in the estuary also appear to have increased. Silica te concentrations have remained constant with time. The results of II surveys of the estuary completed during 1993 are given. TON and silica te behave conservatively in the estuary, with concentrations determine d by those of the fresh water end-member and hence by processes within the catchment. Phosphate and ammonia concentrations at low salinity a re affected by the discharge from the Ellon sewage treatment facility at the head of the estuary. Throughout the estuary there is evidence o f an input of ammonia to the water column from the sediments, but it r epresents only ca 0.2-2% of the nitrogen entering from the River Ythan . Despite the increased nutrient loading to the estuary there is no ev idence that this has resulted in higher chlorophyll concentrations in the water column, this observation is attributed to the relatively sho rt flushing period of the estuary.