Impact of different forcing factors on N:P balance in a semi-enclosed bay: The Gulf of Trieste (North Adriatic Sea) |
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Authors: | M Lipizer G Cossarini |
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Institution: | a Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Borgo Grotta Gigante 42/C - 34010 Sgonico, Trieste, Italy b Department of Life Sciences, University of Trieste, Via A. Valerio, 28/A - 34127 Trieste, Italy |
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Abstract: | The availability and partition of nitrogen (N) and phosphorus (P) in inorganic and organic compartments, as well as their stoichiometric ratio, are influenced by both physical and biological forcing factors. On this basis, the temporal and spatial dynamics in N:P atomic ratios in different compartments may provide information on the functioning of marine ecosystems. Here we explore the relative importance of water temperature, river inputs, wind mixing, stratification, ingression of nutrient-depleted Eastern Adriatic Current and phytoplankton biomass on concentrations and ratios between nitrogen and phosphorus in a semi-enclosed bay (the Gulf of Trieste), using data from monitoring programs carried out during 8 years. Water samples are first classified in 6 water types based on N:P ratios in different components, and then relationships between water type space-time distribution and a set of forcing factors is sought. Results show that the gulf is characterised by relatively stable N:P ratios in all compartments (about 23-26), always exceeding the classical Redfield ratio. In the surface layer, however, nitrogen and phosphorus dynamics are decoupled because of river input and plankton productivity, and a significant spatial and temporal variability is observed in terms of stoichiometric balance, nutrient concentrations and partition among the different pools. Deviations from stable N:P ratios follow a seasonal evolution. In spring, continental inputs alter inorganic nutrient compartments (N:P up to 115); later on, during the seasonal succession of biological processes (e.g. late spring phytoplankton blooms, summer increase in microbial activities and autumn phytoplankton blooms), a change is also seen in the organic dissolved and particulate pools. Multivariate statistical analysis suggests that, among the considered forcing factors, the most relevant in modulating the N:P stoichiometry in the Gulf of Trieste are river inputs and ingression of the Eastern Adriatic Current (acting in opposite directions) along with phytoplankton dynamics. During the whole period, besides variations in N:P stoichiometry, in the Gulf of Trieste dissolved organic matter represents the largest pool of N and P, which can provide a source of nutrients for the planktonic community alternative to inorganic nutrient. |
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Keywords: | DIN dissolved inorganic nitrogen DIP dissolved inorganic phosphorus DON dissolved organic nitrogen DOP dissolved organic phosphorus EAC Eastern Adriatic Current N nitrogen NH4 ammonium NO2 nitrite NO3 nitrate P phosphorus PN particulate nitrogen PO4 orthophosphate PP particulate phosphorus TDN total dissolved nitrogen TDP total dissolved phosphorus TN total nitrogen TP total phosphorus |
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