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- Co-limitation of phytoplankton by N and P in a shallow coastal lagoon (Ria Formosa): implications for eutrophication evaluationPublication . Domingues, Rita B.; Nogueira, Patricia; Barbosa, Ana B.The Ria Formosa coastal lagoon is a highly productive shallow ecosystem in southern Portugal, subjected to nutrient inputs from anthropogenic and natural sources. Nutrients are major abiotic drivers of phytoplankton in this system, but their effects on phytoplankton assemblages and the occurrence of nutrient limitation are still poorly understood. The main goal of this study was, thus, to evaluate the occurrence, type, and effects of nutrient limitation on phytoplankton community and specific functional groups in the Ria Formosa coastal lagoon. We conducted nutrient enrichment experiments with factorial additions of nitrogen (N) and phosphorus (P) using natural phytoplankton assemblages from distinct locations in the Ria Formosa, throughout a yearly cycle. Phytoplankton composition and abundance were evaluated using inverted and epifluorescence microscopies, and spectrophotometric methods were used for biomass. Limitation was defined as higher phytoplankton growth following enrichment with a particular nutrient in relation to the non-enriched control. The most common type of phytoplankton limitation was simultaneous co-limitation by N and P; diatoms, as r-strategists, were the most frequently limited group. Single N and P limitation, and serial P limitation were also observed, as well as negative responses to nutrient enrichment. Group-specific responses to nutrient enrichment were not reflected in the relative abundance of phytoplankton groups within the whole assemblage, due to the numerical dominance of pico-sized groups (cyanobacteria and eukaryotic picophytoplankton). Ambient nutrient ratios and concentrations did not predict phytoplankton nutrient limitation, given the different nutrient utilisation traits among phytoplankton functional groups. Therefore, nutrient ratios should not be used as indicators of nutrient limitation in eutrophication assessment.
- Nutrient limitation of phytoplankton growth in the freshwater tidal zone of a turbid, Mediterranean estuaryPublication . Domingues, Rita B.; Anselmo, Tânia P.; Barbosa, Ana B.; Sommer, Ulrich; Galvão, Helena M.Identification of the limiting nutrient(s) is a requirement for the rational management of eutrophication. Here, we present the first experimental analysis of nutrient limitation of phytoplankton growth and its seasonal variation in the Guadiana estuary (SE Portugal-SW Spain). Ten microcosm experiments were performed during 2005 and 2008, using water samples collected in the freshwater tidal zone of the Guadiana estuary. Nitrate, phosphate and silicate were added in a single pulse, alone and in combinations. Experimental treatments were incubated for 4 days under controlled laboratory conditions. Phytoplankton response to nutrient enrichment was evaluated through changes in biomass (Chla), and abundance of specific phytoplankton groups. Overall, phytoplankton growth seemed to be nitrogen-limited throughout the productive period, especially green algae in 2005 and diatoms in 2008. In the summer 2008, cyanobacteria and the harmful dinoflagellate Kryptoperidinium foliaceum responded to N enrichment in the absence of Si. Indeed, the presence of K. foliaceum was observed for the first time in the freshwater tidal reaches of the Guadiana estuary, where dinoflagellates were usually absent or rare. The significant increase on dinoflagellates and cyanobacteria growth in response to N enrichment in the absence of Si is alarming, because anthropogenic nutrient enrichments usually increase N and P, but not Si. Furthermore, relatively high N concentrations, up to 22 mM, were found to be limiting to phytoplankton growth. These results should therefore be used as a management tool when establishing nutrient criteria and nutrient loading budgets to estuarine waters.
- Are nutrients and light limiting summer phytoplankton in a temperate coastal lagoon?Publication . Domingues, Rita B.; Guerra, Cátia C.; Barbosa, Ana B.; Galvão, HelenaThe Ria Formosa coastal lagoon is one of the most important and vulnerable ecosystems in Portugal, and it is subjected to strong anthropogenic pressures and natural nutrient inputs associated with coastal upwelling. The aim of this study was to evaluate the occurrence of nutrient and light limitation of phytoplankton growth during the productive period, and assess potential impacts of limitation on ecosystem eutrophication. Inorganic nutrients were added to natural microcosms filled with water collected at the landward and seaward boundaries, in summer 2012. Experimental treatments were incubated in situ under two different light intensities during 24 h. Phytoplankton composition, abundance and biomass, net growth rates and nutrient consumption were evaluated. At the landward location, potential nutrient limitation by nitrogen was observed. Nitrogen addition led to a significant increase in N consumption, resulting in higher phytoplankton growth, mainly diatoms, in all N-enriched treatments, under both light intensities. Significant consumption of silica and phosphorus was not reflected on growth, and it was probably due to luxury consumption. At the seaward station, hytoplankton, mainly cyanobacteria and eukaryotic picophytoplankton, were primarily limited by light, due to a deeper mixed layer. Nutrients were not limiting the phytoplankton growth due to import of nutrients from upwelled waters to the adjacent coastal zone.
- Will nutrient and light limitation prevent eutrophication in an anthropogenically-impacted coastal lagoon?Publication . Domingues, Rita B.; Guerra, Cátia C.; Barbosa, Ana B.; Galvão, Helena M.The Ria Formosa coastal lagoon (southern Portugal) is a highly productive and valuable temperate ecosystem, subjected to strong anthropogenic impacts and highly vulnerable to climate change. The main goal of this work is to understand ecosystem susceptibility to eutrophication, by evaluating the isolated and combined effects of nutrient (N, P and Si) and light enrichments on phytoplankton growth and community composition during autumn, winter and spring. Microcosms of natural phytoplankton collected in the lagoon were subjected to different nutrient and light treatments and incubated in situ for 48 h. Nutrient consumption, and phytoplankton growth and community structure were evaluated using spectrophotometric methods, and inverted and epifluorescence microscopy. Diatoms were the only group potentially limited by nitrogen, and only during spring. Increased nutrient consumptions were observed for all nutrient additions in all seasons, not associated with phytoplankton growth, suggesting that luxury consumption was used to build up intracellular nutrient pools. Responses to light enrichment were inconsistent among phytoplankton groups, probably due to a high taxonomic seasonal variability. Positive responses to light enrichment were mostly observed during winter. Negative synergistic interactions between nutrients and light were also observed. We conclude that eutrophication is currently not a problem in the Ria Formosa coastal lagoon, but future nutrient enrichments may lead to accelerated growth of specific functional groups and species, if light is not limiting.