Hypernutrification in the Oligotrophic Eastern Mediterranean: A study in Haifa Bay (Israel)

This study presents the results of six cruises surveying salinity, temperature, 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 characteristics in the bay. The spatial distribution of nutrients in the bay was determined by two main processes: (1) introduction of nutrient-rich waters through the Kishon estuary and their mixing with open seawater; and (2) utilization of nutrients by plankton. The immediate effect of the introduction of riverine waters into the bay was hypernutrification and the enhancement of primary productivity. Although the localized influence close to the estuary was elevated, it faded off drastically seaward to normal oligotrophic conditions. Nutrient concentrations in the bay were similar among all seasons. o-PO₄ ranged between 0.02 and 29 μM, NO₃+NO₂ between below 0.05 and 30 μM, Si(OH)₄ between 0.2 and 18 μM and NH₄ between below 0.1 and 38.0 μM. Chlorophyll a concentrations, on the other hand, were seasonally dependent. Lowest chlorophyll a concentrations (between 0.2 and 3 μg^-1) were found during winter, when productivity was inhibited by low temperature, while the highest concentrations (up to 48 μg l^-1) were found during 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. Median 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 probably the direct consequence of the introduction of riverine water with high o-PO₄ concentrations (low N:P ratios) to the bay. During three instances, when the highest productivity was observed, probable Si limitation was also found, that was shown elsewhere to cause changes in phytoplankton population. Salinity vs o-PO₄ and salinity vs N mixing lines were seasonally dependent, with no significant correlation in the winter. The mixing lines were highly significant in the summer when the outflow of the Kishon was constituted mainly of industrial and domestic effluents undiluted by fresh water. Theoretical chlorophyll a concentrations, calculated from the inorganic N vs salinity mixing diagrams, agreed with the measured values during the natural plankton bloom in the spring. A strong effort is now in progress to clean up the fiver and diminish its adverse influence on the bay.