ECOLOGY OF AMORPHOUS AGGREGATIONS (MARINE SNOW) IN THE NORTHERN ADRIATIC SEA .4. DISSOLVED NUTRIENTS AND THE AUTOTROPHIC COMMUNITY ASSOCIATED WITH MARINE SNOW

In the Northern Adriatic Sea, development over time of the inorganic nutrient regime as well as the autotrophic community was investigated in marine snow (up to 20 cm in diameter) and in clouds (up to 5 m in length) in summer 1991. Inorganic nitrogen species did not follow a particular trend over the investigation period. In marine snow mean enrichment factors (EFs) for inorganic nitrogen species ranged from 2 (NH3) to 3.5 (NO2-) while for PO43- a mean EF of 27 was obtained. Clouds exhibited generally similar EFs. In ambient water, the mean atomic ratio of N(inorg):P-PO43- was ca 70 and therefore deviated significantly from the Redfield ratio of 16. This severe deficiency in phosphate availability might be one of the triggering factors for excessive production of mucoid material by phytoplankton. The mean ratio of N(inorg):P-PO43- was 13 in marine snow, and 9 in clouds, indicating a tendency towards increasing nitrogen limitation for primary production as marine snow ages. During the investigation period, cyanobacteria contributed ca 75 % to the total chl a concentration in ambient water and 68 % in marine snow. Particulate primary production in marine snow (integrated over several irradiance levels) was about 10 times higher (12.45 mug C l-1 h-1) than in ambient water and similar to the production in clouds while specific primary production in marine snow was significantly lower than in ambient water. In clouds specific productivity was ca 1 order of magnitude lower than in ambient water. Based on our measurements, we calculated that the inorganic nitrogen pool is turned over 5 times faster in marine snow than in ambient water while inorganic phosphorus is turned over twice as fast in ambient water. The estimated contribution of primary production associated with marine snow and clouds to overall water column primary production amounts to ca 40 %.