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  • Are microcosm volume and sample pre-filtration relevant to evaluate phytoplankton growth?
    Publication . Nogueira, Patrícia; Domingues, Rita B.; Barbosa, Ana B.
    “Bottle effects” are one of the most deeply rooted concerns of phytoplankton microcosm studies and are mainly related to incubation time and sample volume. Sample pre-filtration to remove larger grazers is also a common procedure in experimental phytoplankton ecology studies, particularly in nutrient enrichment experiments. However, the effects of bottle volume and sample pre-filtration on the outcomes of such experiments, particularly on the net growth rates of specific phytoplankton taxa, have never been addressed. Therefore, this study aims to evaluate the effects of different bottle volumes and sample pre-filtration on phytoplankton net growth rates in microcosm experiments. To accomplish this goal, unfiltered and filtered (b100 μm)water samples, collected in the Guadiana estuary,were nutrient-enriched to avoid nutrient limitation and incubated for 3 days in polycarbonate microcosms with different volumes (0.5 L–8.0 L), inside a plant growth chamber. Phytoplankton composition, abundance, biomass and taxon-specific net growth rates were evaluated throughout the experiment. No systematic significant effects of bottle volume were detected in phytoplankton growth rates. However, sample filtration caused significant changes in phytoplankton composition, with a decline of diatom abundance. Moreover, the removal of large-sized predators and large-sized phytoplankton (diatoms) after sample filtration cascaded down the food web, affecting taxon-specific net growth rates differently. Net growth rates of green algae and eukaryotic picophytoplankton were significantly higher in filtered treatments in respect to unfiltered treatments. Conversely, both diatoms and cryptophytes presented higher net growth rates in unfiltered treatments while net growth rates of picoplanktonic cyanobacteria and plastidic nanoflagellates were not affected by sample filtration.We conclude that, while microcosm volume does not affect results in phytoplankton microcosms, sample pre-filtration may significantly alter the structure of the original phytoplankton community and hence increase the problems associated with the extrapolation of experimental outcomes to the natural environment.
  • Co-limitation of phytoplankton by N and P in a shallow coastal lagoon (Ria Formosa): implications for eutrophication evaluation
    Publication . 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.
  • Tidal variability of phytoplankton and their environmental drivers in the freshwater reaches of the Guadiana estuary (SW Iberia)
    Publication . Domingues, Rita B.; Anselmo, Tânia P.; Barbosa, Ana B.; Sommer, Ulrich; Galvão, Helena M.
    The effects of different phases of semidiurnal and spring-neap tidal cycles on phytoplankton and environmental drivers were evaluated in a tidal, freshwater location of a mesotidal estuary (Guadiana estuary, SW Iberia). An Eulerian approach was used and sampling covered different seasons during 2008. Samples were collected during spring and neap tides, at high tide, mid-ebb, low tide and mid-flood. Several physical-chemical variables were measured, as well as phytoplankton abundance and biomass. Salinity was higher at high-tide and suspended particulate matter was higher during spring tides and flood, due to higher vertical mixing and resuspension of bottom sediments. Chlorophyll a concentration during winter and summer neap tides was higher than during spring tides, whilst the abundance of pennate diatoms was higher during winter and Spring spring tides than during neap tides, probably reflecting differences in river discharge. Overall, tidally-induced differences detected in the freshwater tidal reaches of the Guadiana estuary were not as considerable as those observed in the lower estuary. However, the occurrence of tidally-induced variability in some seasons reflects that thorough sampling programs to study estuarine tidal dynamics should be conducted throughout the year. Occasional sampling will not reflect the typical variability of these highly dynamic systems.
  • Light as a driver of phytoplankton growth and production in the freshwater tidal zone of a turbid estuary
    Publication . Domingues, Rita B.; Anselmo, Tânia P.; Barbosa, Ana B.; Sommer, Ulrich; Galvão, Helena M.
    Light is usually the main driver of phytoplankton growth in turbid estuaries, but it has received far less attention than nutrients as a bottom-up factor. This study presents the first experimental analysis of light limitation of phytoplankton growth and production and its seasonal variability in the freshwater tidal reaches of the turbid Guadiana estuary, SE Portugal/SW Spain. Natural phytoplankton communities were exposed to different photosynthetically active radiation (PAR) intensities. Short-term incubations with addition of 14HCO3 were used to estimate photosynthetic parameters and long-term incubations allowed the evaluation of the effects of light on phytoplankton composition and growth. Light limitation of phytoplankton growth occurred throughout the year in the freshwater tidal reaches of the estuary and no photoinhibition was observed at least up to 615 mmol photonsm 2 s 1. In the summer, co-limitation by nutrients prevented a positive response of phytoplankton to light enrichment. Diatoms were the most light-limited group, whilst cyanobacteria were the only group acclimated to low-light conditions. Green algae and dinoflagellates responded positively to higher PAR exposures. High saturating irradiances, high light-saturated rates of primary production and low photosynthetic efficiencies suggest that phytoplankton community was not acclimated to the low-light conditions that prevail in the Guadiana estuary.
  • Nutrient limitation of phytoplankton growth in the freshwater tidal zone of a turbid, Mediterranean estuary
    Publication . 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.
  • Connections between upwelling patterns and phytoplankton variability under different coastal regimes in SW Iberia Peninsula
    Publication . Krug, Lilian; Silvano, Kathleen M.; Barbosa, Ana B.; Domingues, Rita B.; Galvão, Helena M.; Luis, Joaquim; Platt, Trevor; Relvas, Paulo; Sathyendranath, Shubha
    The region off southwestern Iberia (NE Atlantic) encompasses a wide variety of oceanographic regimes, including differently (geographic) oriented coastal areas impacted by upwelling, riverine inputs and submarine groundwater discharge, submarine canyons and seamounts, and open ocean waters, thereby potentially promoting zone-specific phytoplankton dynamics. Overall, this heterogeneous region is classified as being very sensitive to climate change, and climate-driven alterations (e.g., sea surface warming, changes in upwelling patterns and intensity) have been recently reported for the area. The present study aims to understand the contribution of upwelling to seasonal and interannual variability of coastal phytoplankton, using a remote sensing-based approach. Phytoplankton variability was evaluated using satellite-derived chlorophyll-a (Chl-a), as a proxy for phytoplankton biomass, and primary productivity (PP). Chl-a were obtained from merged SeaWiFS (Seaviewing Wide Field-of-view Sensor), MeRIS (Medium Resolution Imaging Spectrometer) and MODIS-Aqua (Moderate-resolution Imaging Spectroradiometer) sensors at Globcolour portal. PP data at 9.25 km resolution were derived from Eppley’s Vertically Generalized Production Model, based on SeaWiFS and MODIS-Aqua and available at the Ocean Productivity site. Upwelling intensity was estimated using the difference in sea surface temperature (SST) between off and nearshore zones. Advanced Very-High Resolution Radiometer (AVHRR) 4 km SST were obtained from Pathfinder database. Other phytoplankton environmental drivers, such as local (e.g., river flow) and global (e.g., North Atlantic Oscillation - NAO) climate variables, were also analysed. The study area was divided into subareas differently impacted by upwelling and riverine flow, and satellitederived data was averaged for each zone. Seasonal and interannual variability covering a 14-year time series (1998- 2011) for each variable/region were explored. Chl-a at offshelf locations was significantly lower than coastal areas, and exhibited a fairly stable unimodal annual cycle, with maximum during March. Coastal locations displayed more variable annual patterns, with spring and summer Chl-a maxima, reflecting the impact of upwelling events and freshwater inputs. In respect to interannual variability, NAO index and coastal Chl-a were negative and significantly correlated, with 1-month lag. Chl-a interannual trends were also correlated to local climate variables, namely riverine flow for the easternmost coastal zone. The correlation between upwelling intensity and phytoplankton off SW Iberia is region-dependent being less strong within regions dominated by riverine influence.
  • Environmental forcing of phytoplankton in a Mediterranean estuary (Guadiana Estuary, southwestern Iberia): a decadal of anthropogenic and climatic influences
    Publication . 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.
  • Repply to: Brito, A.C., Brotas, V., Caetano, M., Coutinho, T.P., Bordalo, A.A., Icely, J., Neto, J.M., Serôdio, J., Moita, T. (2012) "Defining phytoplankton class boundaries in Portuguese transitional waters: an evaluation of the ecological quality status according to the Water Framework Directive" [Ecological Indicators 19, 5-14]
    Publication . Domingues, Rita B.; Barbosa, Ana B.; Galvão, Helena M.
    We write to comment on the recently published paper “Defining phytoplankton class boundaries in Portuguese transitional waters: an evaluation of the ecological quality status according to the Water Framework Directive” (Brito et al., 2012). This paper presents an integrated methodology to analyse the ecological quality status of several Portuguese transitional waters, using phytoplanktonrelated metrics. One of the systems analysed, the Guadiana estuary in southern Portugal, is considered the most problematic estuary, with its upstream water bodies classified as Poor in terms of ecological status. We strongly disagree with this conclusion and we would like to raise awareness to some methodological constraints that, in our opinion, are the basis of such deceptive conclusions and should therefore not be neglected when using phytoplankton to assess the ecological status of natural waters.
  • Short-term interactive effects of ultraviolet radiation, carbon dioxide and nutrient enrichment on phytoplankton in a shallow coastal lagoon
    Publication . Domingues, Rita B.; Guerra, Cátia C.; Galvão, Helena M.; Brotas, Vanda; Barbosa, Ana B.
    The main goal of this study was to evaluate short-term interactions between increased CO2, UVR and inorganic macronutrients (N, P and Si) on summer phytoplankton assemblages in the Ria Formosa coastal lagoon (SW Iberia), subjected to intense anthropogenic pressures and highly vulnerable to climate change. A multifactorial experiment using 20 different nutrient-enriched microcosms exposed to different spectral and CO2 conditions was designed. Before and after a 24-h in situ incubation, phytoplankton abundance and composition were analysed. Impacts and interactive effects of high CO2, UVR and nutrients varied among different functional groups. Increased UVR had negative effects on diatoms and cyanobacteria and positive effects on cryptophytes, whereas increased CO2 inhibited cyanobacteria but increased cryptophyte growth. A positive synergistic interaction between CO2 and UVR was observed for diatoms; high CO2 counteracted the negative effects of UVR under ambient nutrient concentrations. Nutrient enrichments suppressed the negative effects of high CO2 and UVR on cyanobacteria and diatoms, respectively. Beneficial effects of CO2 were observed for diatoms and cryptophytes under combined additions of nitrate and ammonium, suggesting that growth may be limited by DIC availability when the primary limitation by nitrogen is alleviated. Beneficial effects of high CO2 and UVR in diatoms were also induced or intensified by ammonium additions.
  • Phytoplankton composition, growth and production in the Guadiana estuary (SW Iberia): unraveling changes induced after dam construction
    Publication . 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.