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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.
- Effects of nutrient and light enrichment on phytoplankton growthPublication . Domingues, Rita B.; Barbosa, Ana B.Alterations of freshwater flow regimes and increasing eutrophication lead to alterations in light availability and nutrient loading into adjacent estuaries and coastal areas. Phytoplankton community respond to these changes in many ways. Harmful phytoplankton blooms, for instance, may be a consequence of changes in nutrient supply, as well as the replacement of some phytoplankton species (like diatoms, that contribute for the development of large fish and shellfish populations) by ohers (like cyanobacteria, that may be toxic and represent an undesirable food source for higher trophic levels). Nutrient and light enrichment experiments allow us to understand and predict the effects of eutrophication on the growth of phytoplankton. This is a fundamental tool in water management issues, since it enables the prediction of changes in the phytoplankton community that may be harmful to the whole ecosystem, and the design of mitigation strategies (Zalewski 2000).
- Environmental forcing of phytoplankton in a Mediterranean estuary (Guadiana Estuary, southwestern Iberia): a decadal of anthropogenic and climatic influencesPublication . Barbosa, Ana B.; Domingues, Rita B.; Galvão, Helena M.Phytoplankton seasonal and interannual variability in theGuadiana upper estuarywas analyzed during 1996–2005, a period that encompassed a climatic controlled reduction in river flow that was superimposed on the construction of a dam. Phytoplankton seasonal patterns revealed an alternation between a persistent light limitation and episodic nutrient limitation. Phytoplankton succession, with early spring diatom blooms and summer–early fall cyanobacterial blooms, was apparently driven by changes in nutrients, water temperature, and turbulence, clearly demonstrating the role of river flow and climate variability. Light intensity in the mixed layer was a prevalent driver of phytoplankton interannual variability, and the increased turbidity caused by the Alqueva dam construction was linked to pronounced decreases in chlorophyll a concentration, particularly at the start and end of the phytoplankton growing period. Decreases in annual maximum and average abundances of diatoms, green algae, and cyanobacteria were also detected. Furthermore, chlorophyll a decreases after dam filling and a decrease in turbidity may point to a shift from light limitation towards a more nutrient-limited mode in the near future.
- Phytoplankton composition, growth and production in the Guadiana estuary (SW Iberia): unraveling changes induced after dam constructionPublication . Domingues, Rita B.; Barbosa, Ana B.; Sommer, Ulrich; Galvão, Helena M.Water quality and quantity problems in the Guadiana estuary due to a recently built dam have been predicted, including an enhancement of cyanobacteria blooms. The main goal of this work was thus to describe the present phytoplankton dynamics in relation to its environmental drivers and to evaluate the effects of damming on phytoplankton in the Guadiana estuary. Sampling campaigns were conducted during 2007–2009 in 4 locations of the Guadiana estuary, covering the salinity gradient. Phytoplankton-related and physical–chemical variables were analyzed. Throughout our study, light availability was mainly controlled by suspended sediments and it was much lower than saturating intensities described for phytoplankton growth. Therefore, light was probably limiting to phytoplankton growth throughout the year, especially in the middle and upper estuarine zones. Nitrogen limitation of phytoplankton growth occurred occasionally throughout the study period, especially during spring and summer. Overall, light and nutrient availability were mainly controlled by river flow; anthropogenic sources of nutrients to the estuary were negligible. Phytoplankton showed a unimodal cycle with biomass maximum in late spring/early summer, and the typical seasonal succession of freshwater phytoplankton (diatoms, green algae, cyanobacteria) was observed. Diatoms were the main component of the phytoplankton community and their variability closely followed nitrate and river flow variability. The relative abundance of the main phytoplankton groups changed in relation to the period before dam construction, with a decrease on cyanobacteria contribution to total abundance. The environmental perturbation induced by dam construction has now stabilized and resulted in an overall decrease in nutrient concentrations, an increase in light availability and a decrease in cyanobacteria abundance.
- 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.