The environmental conditions of semi-enclosed coastal water-bodies are directly related to the catchment, human activities, and oceanographic setting in which they are located. As a result of low tidal forcing, and generally weak currents, waters in Mediterranean harbours are poorly renewed, leading to quality deterioration. Here, we characterise the seasonal variation of trace metals (i.e. Co, Cd, Cu, Fe, Mo, Ni, Pb, and Zn) in surface waters, and trace metal content in sediments from Maó Harbour, a semi-enclosed coastal ecosystem in the NW Mediterranean Sea. Our results show that most of the dissolved trace metals in the waters of Maó Harbour exhibit a marked inner-outer concentration gradient, suggesting a permanent input into the inner part of the harbour. In general, metal concentrations in the waters of Maó Harbour are higher than those in offshore waters. Concentration of Cu (21±8nM), Fe (9.2±3.2nM) and Pb (1.3±0.4nM) are particularly high when compared with other coastal areas of the Mediterranean Sea. The concentration of some metals such as Cu and Zn increases during summertime, when the human population and boat traffic increase during the tourism season, and when resuspension from the metal enriched sediments is higher. The evaluation of the metal sources in the harbour reveals that, compared with other putative sources such as runoff, aerosol deposition and fresh groundwater discharges, contaminated sediments are the main source of the metals found in the water column, most likely through vessel-driven resuspension events. This study contributes to the understanding of the processes that control the occurrence and distribution of trace metals in Maó Harbour, thus aiding in the effective management of the harbour, and enhancing the overall quality of the seawater ecosystem.

We investigated the species-specific phosphorus (P) nutritional strategies in the microphytoplankton community in the Mahon estuary (Minorca, Western Mediterranean) in 2011, under two contrasting hydrographic scenarios. Estuarine flow, nutrient concentrations, phytoplankton community composition, and enzyme-labeled fluorescence (ELF) were measured in June and October, corresponding to the beginning and the end of summer. Dissolved inorganic nitrogen (DIN) and inorganic phosphate (P i ) exhibited enhanced concentrations in the inner estuary where N:P molar ratios suggested P-limitation in both surveys. P i was low and variable (0.09 ± 0.02 µmol L −1 in June and 0.06 ± 0.02 µmol L −1 in October), whereas organic phosphorus remained a more readily available P source. Even though ambient P i concentrations were slightly higher on June, when the microphytoplankton assemblage was dominated by dinoflagellates, the percentage of ELF labeled cells was notably higher (65 ± 9% of total cells) than in October (12 ± 10%), when the presence of diatoms characterized the microphytoplankton community. Alkaline phosphatase activity (AP) was mainly expressed by dinoflagellate taxa, whereas diatoms only displayed significant AP in the inner estuary during the June survey. A P-addition bioassay in which response of AP to P i enrichment was evaluated showed marked reduction in AP with increasing P i . However, some dinoflagellate species maintained AP even when P i was supplied in excess. We suggest that in the case of some dinoflagellate species AP is not as tightly controlled by ambient P i as previously believed. AP activity in these species could indicate selective use of organic phosphorus, or slow metabolic response to changes in P forms, rather than physiological stress to low P i availability. We emphasize the importance of identifying the species-specific phosphorus requirements and their capability for P assimilation in order to understand the contribution of microphytoplankton to the whole community response to biogeochemical perturbations.