HYPERNUTRIFICATION IN THE OLIGOTROPHIC EASTERN MEDITERRANEAN - A STUDY IN HAIFA BAY (ISRAEL)

This study presents the results of six cruises surveying salinity, tem perature, nutrients, dissolved oxygen and chlorophyll a concentrations in Haifa Bay, Israel, covering all seasons. The outflow of the Kishon stream, in conjunction with the hydrographical conditions in the bay, are the main factors influencing water quality and the chemical chara cteristics in the bay. The spatial distribution of nutrients in the ba y was determined by two main processes: (1) introduction of nutrient-r ich waters through the Kishon estuary and their mixing with open seawa ter; and (2) utilization of nutrients by plankton. The immediate effec t of the introduction of riverine waters into the bay was hypernutrifi cation and the enhancement of primary productivity. Although the local ized influence close to the estuary was elevated, it faded off drastic ally seaward to normal oligotrophic conditions. Nutrient concentration s in the bay were similar among all seasons. o-PO4 ranged between 0.02 and 29 mu M, NO3 + NO2 between below 0.05 and 30 mu M, Si(OH)(4) betw een 0.2 and 18 mu M and NH4 between below 0.1 and 38.0 mu M. Chlorophy ll a concentrations, on the other hand, were seasonally dependent. Low est chlorophyll a concentrations (between 0.2 and 3 mu g l(-1)) were f ound during winter, when productivity was inhibited by low temperature , while the highest concentrations (up to 48 mu g l(-1)) were found du ring the spring. The productivity in the bay was found to be mostly N limited as opposed to the P limitation found in the open sea area. Med ian N:P ratios were similar during the whole year and ranged between 0 .4 and 3.8, much lower than the Redfield's ratio and the N:P ratio of c. 26 found in the open sea area. The N limitation in Haifa Bay was pr obably the direct consequence of the introduction of riverine water wi th high o-PO4 concentrations (low N:P ratios) to the bay. During three instances, when the highest productivity was observed, probable Si li mitation was also found, that was shown elsewhere to cause changes in phytoplankton population. Salinity vs o-PO4 and salinity vs N mixing l ines were seasonally dependent, with no significant correlation in the winter. The mixing lines were highly significant in the summer when t he outflow of the Kishon was constituted mainly of industrial and dome stic effluents undiluted by fresh water. Theoretical chlorophyll a con centrations, calculated from the inorganic N vs salinity mixing diagra ms, agreed with the measured values during the natural plankton bloom in the spring. A strong effort is now in progress to clean up the rive r and diminish its adverse influence on the bay. (C) 1998 Academic Pre ss Limited.