NUTRIENTS LIMITING THE ALGAL GROWTH-POTENTIAL (AGP) IN THE PO RIVER PLUME AND AN ADJACENT AREA, NORTHWEST ADRIATIC SEA - ENRICHMENT BIOASSAYS WITH THE TEST ALGAE NITZSCHIA-CLOSTERIUM AND THALASSIOSIRA-PSEUDONANA

From April 1993 to March 1994, 135 samples were collected at two sites in the coastal Adriatic Sea, both near (14.5 km) and far (63 km) from the Po River delta. The nutrient(s) limiting algal growth potential ( AGP) were estimated by bioassay using Nitzschia closterium (local isol ate) and Thalassiosira pseudonana (strain 3-H). Estimates were also ma de by comparing the nutrient molar ratios, Sigma N:P and Sigma N:Si (w here Sigma N = NO3- + NO2- + NH4+ + urea), to the Redfield Ratio (16:1 , 1:1). According to the bioassay results, phosphorus was the sole nut rient limiting AGP in 2% of the samples and was the most limiting nutr ient in 69% of the samples; nitrogen and phosphorus were equally co-li miting. Omission of phosphorus from spike enrichments allowed, on aver age, only a 1.6-fold increase in biomass over that in the unenriched c ontrols. Similar omission of nitrogen allowed a 4-fold increase, while silicon, iron, and micronutrients resulted in 14-fold, 18-fold, and > 20-fold increases, respectively. In most of the samples, Sigma N:P was much greater than 16, indicating a marked phosphorus deficiency, whil e Sigma N:Si values suggested that silicon was the third most limiting nutrient in 35% of samples. In water collected far from the Po delta, the yield of N. closterium was not limited by any nutrients other tha n the three major ones: P, N and Si. In these same waters, T pseudonan a was also potentially limited by iron and, to a lesser extent, by vit amins. The role of iron varied. In samples collected near the Po delta , iron acted as the third most limiting nutrient for N. closterium in June and September; it appeared 29 times out of 78 on the list of pote ntially limiting nutrients for T. pseudonana, including 5 times as the most limiting. Altogether, comparison with published results suggests that the roles of iron and silicon in AGP limitation have increased d uring the past three decades, and could become even more important if eutrophication in the Adriatic Sea continues to increase.