Seasonal and spatial variations in the nutrient limitation of bacterioplankton growth in the northwestern Mediterranean

Nutrient limitation of bacterioplankton growth was studied in the western Mediterranean Sea to determine its spatial and temporal variations. Shipboard microcosm experiments were performed in June 1995, June 1996 and September 1996. Seawater was amended with carbon (C, as glucose), nitrate (N) and phosphate (P), individually or in combination. The limiting nutrient was inferred from the combination that stimulated the highest bacterial production and generated the highest bacterial biomass in the experiments. The results of the experiments carried out along a coastal-to-open sea transect, with both surface and deep chlorophyll maximum depth waters (DCM), suggest that there is a strong variability in the factors limiting bacteria. While phosphorus was most often the limiting nutrient in the surface samples, at the DCM depths nitrogen or carbon limitation was also found. The surface waters of the open sea station were also studied during the 3 cruises to provide an estimate of seasonal variation of bacterial limitation. Phosphorus limitation was found in the 3 periods: very clear phosphorus limitation in September 1996, possible phosphorus limitation in June 1995 and possible co-limitation with carbon in June 1996. Our results show that phosphorus is the main nutrient limiting bacterioplankton growth in the NW Mediterranean. However, while phosphorus was usually the limiting nutrient in the surface layers, nitrogen and carbon limitation also occurred at other depths. Thus, seasonal and spatial variability in the nutrient limiting bacterial growth should be expected.