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- The relationship between phytoplankton diversity and community function in a coastal lagoonPublication . Duarte, Pedro; Macedo, M.F.; Cancela da Fonseca, LuísThe decrease of biodiversity related to the phenomena of global climate change is stimulating the scientific community towards a better understanding of the relationships between biodiversity and ecosystem functioning. In ecosystems where marked biodiversity changes occur at seasonal time scales, it is easier to relate them with ecosystem functioning. The objective of this work is to analyse the relationship between phytoplankton diversity and primary production in St. André coastal lagoon – SW Portugal. This lagoon is artificially opened to the sea every year in early spring, exhibiting a shift from a marine dominated to a low salinity ecosystem in winter. Data on salinity, temperature, nutrients, phytoplankton species composition, chlorophyll a (Chl a) concentration and primary production were analysed over a year. Modelling studies based on production-irradiance curves were also conducted. A total of 19 taxa were identified among diatoms, dinoflagellates and euglenophyceans, the less abundant group. Lowest diversities (Shannon– Wiener index) were observed just before the opening to the sea. Results show a negative correlation (p<0.05) between diversity and chlorophyll a (Chl a) concentration (0.2–40.3 mg Chl a m-3). Higher Chl a values corresponded to periods when the community was dominated by the dinoflagellate Prorocentrum minimum (>90% of cell abundance) and production was maximal (up to 234.8 mg C m-3 h-1). Maximal photosynthetic rates (Pmax) (2.0–22.5 mg C mg Chl a-1 h-1) were higher under lower Chl a concentrations. The results of this work suggest that decreases in diversity are associated with increases in biomass and production, whereas increases correspond to opposite trends. It is suggested that these trends, contrary to those observed in terrestrial and in some benthic ecosystems, may be a result of low habitat diversity in the water column and resulting competitive pressure. The occurrence of the highest photosynthetic rates when Chl a is low, under some of the highest diversities, suggests a more efficient use of irradiance under low biomass–high diversity conditions. Results suggest that this increased efficiency is not explained by potential reductions in nutrient limitation and intraspecific competition under lower biomasses and may be a result of niche complementarity.
- The influence of incubation periods on photosynthesis–irradiance curvesPublication . Macedo, M.F.; Duarte, Pedro; Ferreira, JoãoIn phytoplankton primary production studies, samples for the determination of photosynthesis versus irradiance relationship ( P–I) are usually incubated at several irradiance levels during a fixed time period, commonly 2–4 h. However, it is not clear if the use of this fixed incubation time is appropriate to study the P–I relationship in any given ecosystem. The aim of this work was to study the influence of incubation time on the P–I relationship in natural phytoplankton populations from three different coastal ecosystems: an open coastal area, an estuary, and a coastal lagoon. Physical and chemical variables, phytoplankton biomass, species composition, and P–I curves were analysed. The results showed that, in the coastal area and in the estuary, P–I relationships were time dependent, whereas in the coastal lagoon different incubation periods produced the same P–I curve. An underestimation of daily primary production, ranging from 13% to 42.5%, was calculated when data from standard incubation times (2–4 h) were used in ecosystems where P–I curves present a dynamic time-dependent behaviour. This work suggests that the response of the P–I curves to the incubation time varies with the characteristics of the ecosystem and is related to the light regime to which phytoplankton cells are adapted. D 2002 Elsevier Science B.V. All rights reserved.
- Analysis of the deep chlorophyll maximum across the Azores FrontPublication . Macedo, M.F.; Duarte, Pedro; Ferreira, João; Alves, M.; Costa, V.Physical, chemical and biological observations made in late July and August 1997 across the Azores Front (37ºN, 32ºW to 32ºN, 29ºW) are presented. The objectives of the study were: (1) to analyse horizontal and vertical profiles of temperature, salinity, density, nutrients and chlorophyll-a (Chl a) of the top 350 m; (2) to identify the main differences in the deep Chl a Maximum (DCM) and hydrographic structure between the water masses that pass north and south of the Azores Front; and (3) to estimate phytoplankton primary production in these water masses. Horizontal and vertical profiles of salinity, temperature, density, nutrients and phytoplankton pigments in the top 350 m were analysed. The Front separates two distinct water types: the 18 ºC ModeWater (18MW) of subtropical origin, and the 15 ºC ModeWater (15MW) of sub-polar origin. Differences in the DCM and hydrographic structure between 18 MW and 15 MW were observed in the contour plots of each section. The average Chl a concentration between 5 and 200 m depth decreased significantly from 15 MW to 18 MW. The same pattern was observed for the Chl a concentration at the DCM depth. A vertical one-dimensional model was used to estimate the phytoplankton primary production in the 15 MW and 18 MW and led to an estimated water column average gross primary productivity (GPP) between 1.08 and 2.71 mg C m
- Analysis of coastal lagoon metabolism as a basis for managementPublication . Duarte, Pedro; Bernardo, João; Costa, Ana; Macedo, M.F.; Calado, Gonçalo; Cancela da Fonseca, LuísThis work was carried out in a shallow eutrophic coastal lagoon (St. André lagoon, SW Portugal) which is artificially opened to the sea each year in early spring. Macrophytes, mainly Ruppia cirrhosa, are keystone species in this ecosystem covering up to 60% of its total area with peak biomasses over 500 g DWm−2. The main objectives were to study ecosystem metabolism, to evaluate the metabolic contribution to the community of the macrophyte stands and their influence in the development of thermal stratification and bottom oxygen depletion. The work combined an experimental and a modelling methodology. The experimental approach included open water, mesocosm and microcosm seasonal experiments. During these experiments several physical, chemical and biological parameters were monitored in the lagoon and in plastic enclosures (mesocosms) for periods of 24 hours. The microcosm experiments followed the light-dark bottle technique. The simultaneous use of these different methodologies allowed the analysis of the contribution of the planktonic and benthic compartments to the ecosystem’s oxygen budget. The modelling work was based on the mathematical simulation of heat and gas exchanges in a vertically resolved water column, under different macrophyte densities. Several simulations were carried out, in order to investigate the importance of the macrophytes in the development of water column stratification and anoxia. The simulation results suggest that macrophytes may greatly influence thermocline and oxycline development. This influence is proportional to their biomass and canopy height. It is suggested that controlled macrophyte biomass removal of up to 25% of available biomass in summer, may be useful in preventing bottom anoxia without compromising benthic net primary production.
- Phytoplankton production modelling in three marine ecosystems—static versus dynamic approachPublication . Macedo, M. F.; Duarte, PedroPhytoplankton productivity is usually determined from water samples incubated at a number of irradiance levels during several hours. The resultant productivity-irradiance (P–E) curves are then used to estimate local and/or global phytoplankton production. However, there is growing evidence that these curves, referred as static, underestimate phytoplankton photosynthesis to a great deal, by assuming a stable response to light over the incubation period. One of the drawbacks of static P–E curves is the overestimation of photoinhibition. In this work, three one-dimensional vertically resolved models were developed as simply as possible, to investigate differences between static and dynamic phytoplankton productivity in three marine ecosystems: a turbid estuary, a coastal area and an open ocean ecosystem. The results show that, when photoinhibition development time is considered (dynamic model), the primary production estimates are always higher than when calculated with the static model. The quantitative importance of these differences varies with the type of ecosystem and it appears to be more important in coastal areas and estuaries (from 21 to 72%) than in oceanic waters (10%). Thus, these results suggest that primary production estimates, obtained under the assumption of a static behaviour response to light, may underestimate the real values of global phytoplankton primary production. Calculations suggest that the quantitative importance of this underestimation may be larger than the global missing carbon sink.