Ana B. Barbosa

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 14HCO 3- 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 μmol photons m -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.

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 (Chl a), 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 μM, 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.

Phytoplankton is the dominant primary producer in aquatic ecosystems and is considered a gauge of ecological condition and change. Some phytoplankton groups, namely diatoms, dinoflagellates, and coccolithophores, produce morphological or chemical fossils that can be used for paleoenvironmental reconstruction. This study aims to review the processes that regulate dynamics in living phytoplankton and to highlight how this knowledge is used in paleoecological studies. The distribution patterns of phytoplankton in present-day aquatic ecosystems are shaped by the interplay between processes that regulate cell growth and cell death. Cell growth and cell death are regulated by the internal environment of phytoplankton (e.g., specific environmental tolerances, resource uptake properties, cell size, density and morphology, alternative nutritional strategies such as mixotrophy or N2 uptake, motility, intracellular storage capacities, grazing resistance properties), and by its external environment. The external environment includes variables dependent on the availability of resources (e.g., light intensity, concentration of CO2 and dissolved inorganic macronutrients and micronutrients, availability of living prey in case of mixotrophs) and variables independent of resources (e.g., temperature, salinity, turbulence, ultraviolet radiation, bioactive compounds, activity of grazers, viruses, and eukaryotic parasites). The importance of recently described loss processes, such as grazing by phagotrophic protists, viral lyses, and programmed cell death, is discussed in the context of its potential impact upon phytoplankton vertical fluxes. Examples of the use of different phytoplankton metrics (e.g. abundance, species composition, species morphology, and elemental composition) to infer contemporaneous as well as past environmental and ecological conditions are critically evaluated.

The European Union Water Framework Directive (WFD), a new regulation aiming to achieve and maintain a clean and well-managed water environment, refers to phytoplankton as one of the biological quality elements that should be regularly monitored, and upon which the reference conditions of water quality should be established. However, the use of phytoplankton as a biological quality element will result in several constraints, which are analyzed in this article with examples from Portuguese waters. Specifically, the establishment of reference conditions of water quality may be difficult in some water bodies for which no historical data exists. The sampling frequency proposed for phytoplankton monitoring does not seem suitable to assess phytoplankton succession, and may preclude the detection of algal blooms. Finally, the use of chlorophyll a as a proxy of phytoplankton biomass and abundance has been proposed by some authors, but it may overlook blooms of pico- and small nanophytoplankton, and overestimate the importance of large microphytoplankton. Furthermore, most studies in Portugal have used only inverted microscopy for phytoplankton observation and quantification; this method does not permit the distinction between autotrophic and heterotrophic cells, especially in samples preserved with Lugol’s solution, and does not allow the observation of smaller-sized cells. Finally, some techniques, such as remote sensing and chemotaxonomic analysis, are proposed to be used as supplements in phytoplankton monitoring programs.

Seasonal changes in freshwater flow, leading to alteration of the nutritional environment and hence affecting phytoplankton composition, will probably be enhanced in the Guadiana estuary (SW Iberia) by the recently built Alqueva dam. The main goal of this study is to assess the relationship between dissolved inorganic macronutrient concentrations and ratios, light availability and phytoplankton succession in the upper estuary of the Guadiana River prior to the completion of the dam. From April to October 2001 three locations along the upper estuary were sampled fortnightly. Several physical and chemical parameters were analysed and phytoplankton composition, abundance and biomass were determined through inverted and epifluorescence microscopy. Phytoplankton showed a uni-modal cycle with a biomass maximum during spring. A relationship between phytoplankton succession and nutrient ratios seemed to exist. In early spring, N:P was high, Si was abundant and a diatom bloom occurred. This bloom collapsed and an increase in green algae abundance was observed later in spring, with low Si and high N:P. In the summer, N:P and Si were low, and a cyanobacteria bloom developed. This bloom included the potentially toxic Microcystis. Light was probably limiting throughout the sampling period, particularly to non-motile cells. Enhancement of cyanobacteria blooms can be expected, and as the river water is used by local human populations, continued monitoring of the Guadiana estuary will be necessary to evaluate the effects of the Alqueva dam construction on phytoplankton dynamics.

The aim of this study was to characterise phytoplankton dynamics in a coastal saline lake, pinpointing putative biotic and abiotic regulatory variables of its succession and productivity. Between February and September 1998, samples for the analysis of physical, chemical and biological variables were taken fortnightly (except in February and April). The phytoplankton community showed three distinct periods of evolution. The first period (February-March) was characterised by a chroococoid non-colonial cyanobacteria bloom (maximum abundance, 4.3 × 109cells l–1) and also by its decaying. Long water residence and/or nitrogen limitation might have allowed cyanobacteria dominance; while its decaying could be associated to predation by aplastidic nanoflagellates and/or to the beginning of periodical partial renewal of lake water with water proceeding from an adjacent coastal lagoon (Ria Formosa). The second period (April-early August) can be differentiated, from the previous, by reduced abundances of phytoplankton (minimum abundance, 5.7 × 106 cells l–1) and plastidic nanoflagellates dominance. The overall low nutrient concentrations, likely as a consequence of periodical partial water renewal, could explain these results. In the last period (late August-September), increased phytoplankton abundance and the development of a diatom and mixotrophic dinoflagellate bloom was probably the result of a sudden increase in nutrient levels, occurring after a period of intense precipitation. In consequence, primary production reached a maximum value of 1367 mg C m–3 h–1; 36 times higher than a maximum value previously reported for Ria Formosa.

Short-term variability of heterotrophic bacterioplankton was studied in a recently upwelled water mass at the NW Iberian margin (August 1998). Bacterioplankton abundance (BA), biomass (BB), production (BP), and specific production (SBP) were monitored during two Lagrangian drift experiments, one along the shelf-edge, the other off-shelf along an upwelling filament. Other measurements included chlorophyll a (Chla), primary production (PP), suspended particulate organic carbon (POC) and nitrogen (PON), and dissolved organic carbon (DOC) and nitrogen (DON). Although primary production was significantly higher during the shelf-edge drift experiment, bacterial biomass in the euphotic zone (2.68 to 22.20μgC.l−1) was not significantly different from that in the offshore filament. In contrast, bacterial production (0.13-3.52μgC.l−1.d−1), estimated using an empirically determined 14C-leucine to carbon conversion factor, and bacterial growth rates (doubling time, DT: 3.9–29.7d), were significantly higher during the shelf-edge drift (BP: 1.50±0.11 versus 0.50±0.02μgC.l−1.d−1; DT: 6.9±0.3 versus 16.2±0.9 d; p<0.01). Depth-integrated BB over the euphotic zone comprised 15±1% of phytoplankton biomass during shelf-edge drift and 39±4% under the more oligotrophic conditions in the filament. However, daily BP to net primary production ratios were not significantly different in the two regions (6±1% versus 7±1%). BA, BB, BP and SBP were enhanced in the later part of the shelf-edge drift following a pronounced increase in both PP and gross DOC production, suggesting that phytoplankton was a source of substrates for bacteria in recently upwelled waters. This contrasted with the filament drift in which short-term variability of bacterioplankton was much less pronounced and there was no correlation between BP and PP. In both regions, SBP and DOC in the euphotic zone were significantly correlated (p<0.005) indicating some regulatory effect of DOC over bacterial activity. Bacterial carbon demand in the euphotic zone ranged from 22.5 to 44.0μmolC.m−3.h−1, and 10.6 to 11.3μmolC.m−3.h−1 and represented 62% and 43% of overall gross DOC production during shelf-edge and filament drift experiments. During the filament drift, the relatively high bacterial doubling times coupled with high percentage of non-metabolised DOC (ca. 60%) suggested control of bacteria by either bioavailability of DOM or inorganic nutrients. Between 40 and 60% of gross DOC production was not metabolised by heterotrophic bacteria, thus allowing DOC net accumulation and off-shore advection.

Water samples were collected at bimonthly intervals (April–October 1988) from three stations located in the Ria Formosa coastal lagoon (SE Portugal). Abundance and biovolume of heterotrophic bacterioplankton ranged between 1.2–18 x 106 cells. ml-1 and ...