Primary production in systems subject to natural and anthropogenic eutrophication

Tese dout., Química, Universidade do Algarve, 2005

The input of nutrients to the aquatic system can have both natural (e.g. upwelling events) and human (e.g. sewage discharges, run-off of inorganic fertilizers from agricultural land) origins. The level of anthropogenic (human) eutrophication has grown in the past century. Primary production rates have increased significantly as a response, which can lead to harmful effects when the balance between the production and decomposition of organic matter is affected. These include the depletion of oxygen as excessive organic matter decomposes and the migration or death of surrounding life forms. Other consequences of nutrient enrichment may include changes in the natural autotrophic community structure by altered nutrient ratios, reduced biological diversity, increase in harmful algal blooms, poisoning of organisms, concluding in deterioration of the ecological and chemical status of waters. A decrease in fisheries, aquaculture and recreational resources may follow. To control and reverse eutrophication it is important to understand the dynamics between nutrients and the algal assemblage, identifying which nutrient is primarily limiting algae biomass and production. Within the context of the European Union (EU) several policies aim to reduce eutrophication and ensure the protection and improvement of the European aquatic environment. These include the Urban Waste Water Treatment (UWWT) Directive (91/271/EEC), the Nitrate Directive (91/676/EEC) and the recent Water Framework Directive (WFD) (2000/60/EC). The general objective of this thesis was to investigate the relation between nutrients (specifically nitrogen and phosphorus) and the activity and structure of the pelagic assemblage of microalgae in the two selected study areas, recently included as part of the intercalibration sites for the Common Implementation Strategy of the WFD. The underlying hypothesis is that the microphytoplankton assemblage is affected by nutrient concentrations and ratios. This was tested both by field and experimental work. In management terms, this study evaluates whether the microphytoplankton assemblage (and its selected supporting elements) are good tools for the identification and management of anthropogenic eutrophication within the context of the WFD. Are these elements sufficient to distinguish between Natural and Anthropogenic Eutrophication? Can we use these tools for management? The Sagres station (SW. Portugal), adjacent to the upwelling centre of Cabo S. Vicente and close to an oyster-culture was selected for the study of natural eutrophication, whereas xv the Ria Formosa coastal lagoon (S. Portugal) was representative of a system subject to both natural and anthropogenic nutrient enrichment. The sampling in Sagres was performed during the upwelling season, from May to September (2001). The Ria Formosa was sampled according to the frequency recommended by the WFD (every 3 months) during representative conditions: close to the summer and winter solstice, and spring and autumn equinox, coinciding with both high (HW) and low water (LW), between June 2001 and July 2002. The water was collected from three contrasting stations on the western lagoon. Nitrogen and phosphorus enrichment experiments were conducted additionally over short-time scales on small volumes of water collected during a period of relaxation of upwelling conditions in Sagres (September 2002), and during the growing season (summer solstice of 2001 and 2002), as well as the autumn equinox (September 2001), in the Ria Formosa. The results indicated that during the upwelling season the cold nutrient-rich waters of Sagres were dominated by the presence of diatoms. Flagellate forms developed by the end of the season when stratification occured, which was probably associated with the intrusion of the warm coastal countercurrent from the Gulf of Cádiz. Chaetoceros spp., Thalassiosira spp., Lauderia spp., Detonula spp., and Pseudo-nitzschia spp. were identified as a summer upwelling proxy. Production rates (seasonal average gross production: 25.4±19.8 μM O2 d-1) were comparable to other productive systems. Respiration, on the contrary, was low during the survey. Nitrogen seemed to be the major nutrient regulating the microalgae activity, both during the upwelling season in the form of “new” nitrogen (essentially nitrate), and during relaxation stages in regenerated forms (essentially ammonium). Diatoms were the most sensitive group to nitrogen enrichment, including Pseudo-nitzschia spp. that may include toxic species. Altogether, the main factors influencing the development and structure of the algal community in Sagres were of physical nature, which in turn determined the chemical supply of nutrients to the photic zone. The microplankton community peaked in the summer solstice samplings (June 2001 and July 2002) in the Ria Formosa lagoon, with diatoms as the greatest contributors to the total numbers, whereas flagellates generally dominated the carbon biomass. The statistical analysis clustered the microplankton samples into their seasonal sampling groups. Physical factors such as temperature, salinity and solar radiation, together with the availability of nitrogen (especially in reduced forms) appear to be important parameters contributing for the microplanktonic evolution. The oceanic assemblage differed from the internal lagoon community on certain occasions. Events such as coastal upwelling in the adjacent waters and land run-off episodes within the Ria can contribute to a greater differentiation between the Primary Production in systems subject to Natural and Anthropogenic Eutrophication xvii Ria-ocean systems, and thereby to the selection of distinct biological forms. The Ria had an important role as an exporter of both algal biomass and nutrients to adjacent waters during the sampling campaigns. The importance of coastal upwelling events as a contributor for the trophic level of the lagoon was also highlighted. Further work may clarify the relative degree of natural versus anthropogenic processes for the eutrophication of the lagoon. High concentrations of nutrients and algal biomass can accumulate in regions prone to restricted water renewal such as upstream channels. High numbers of microalgae can also occur adjacent to UWWT plants. Transport mechanisms ultimately regulate the final location of the accumulated material. Nitrogen is suggested to be the potential limiting nutrient of the pelagic microalgae community of the lagoon during the growing season according to enrichment experiments, with diatoms as the most sensitive group. In temperate ecosystems, a switch in limiting factors is expected due to the seasonal change of biological and environmental variables. The systems appear to differ significantly during the growing season, both in biological and physico-chemical parameters, but these differences seem to be nevertheless not sufficient to distinguish between Natural and Anthropogenic Eutrophication. Similar responses to enrichment bioassays were also observed in both systems. Harmful Algal Blooms (HAB) taxa seem to develop mainly due to natural events (upwelling), suggesting that this parameter should be used with caution as an indicator of anthropogenic eutrophication in Portuguese coastal waters. Altogether, although the fact that the microphytoplankton assemblage responded to changing trophic conditions supports the need to incorporate these data in monitoring programmes, these tools seem not to be sufficient for the management of anthropogenic eutrophication in the selected areas. The two scenarios were in the ranges of mesotrophic conditions during the samplings, which is likely a consequence of the mechanisms of mixing and transport present in both areas. The high potential of dilution from the Atlantic Portuguese coastal waters contributes to its low degree of eutrophication problems. This works contributes to the on-going process of implementation of the WFD, and to the database of knowledge of the productive systems of Sagres and Ria Formosa.

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