Browsing by Author "Tett, Paul"
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- Changes in the yield of microphytobenthic chlorophyll from nutrients: Considering denitrificationPublication . Brito, Ana; Newton, Alice; Tett, Paul; Fernandes, TeresaThe yield of microphytobenthic (MPB) chlorophyll from nitrogen expresses the relationship between chlorophyll formation and nutrient consumption within the surface sediment. In shallow coastal lagoons MPB have an important role in chlorophyll dynamics and may represent the majority of microalgal chlorophyll pigments. Given that chlorophyll has been used as an indicator of eutrophication, the yield is of utmost importance. Furthermore, the yield is a key parameter in models that aims at predicting eutrophication. Estimates of the MPB yield have been previously obtained from a series of microcosm experiments. However, discrepant results in nitrogen fluxes suggested the importance of other processes such as denitrification. Therefore, denitrification rates were used for the calculation of nutrient fluxes and new yield estimates were obtained for muddy sediments. Whereas the earlier estimate obtained was of 4.1 g chl. ( mol N)−1, after including the denitrification rate in the calculation this value was reduced from 5 to 35%.
- Development of an optimal methodology for the extraction of microphytobenthic chlorophyllPublication . Brito, Ana; Newton, Alice; Tett, Paul; Fernandes, TeresaBenthic microalgae are important primary producers in intertidal shallow systems. Their biomass can be estimated by the assessment of chlorophyll a concentration. A rapid and reliable method of measuring chlorophyll a is by spectrophotometer. There is however, no standard protocol for the analysis of benthic chlorophyll a. Although the most common solvent generally used is 90% acetone, some authors showed better results with methanol and ethanol. Some pre-treatments, such as the addition of fine inert granules or ultrasound bath, have also been suggested as factors that improve the extraction efficiency. Sediment samples were collected from two sites, muddy and sandy, located within Ria Formosa (Portugal). The aim of this work was to test the effectiveness of different pre-treatments in the extraction and to develop an optimal method for chlorophyll a extraction and analysis. Pre-treating samples did not yield any significant differences in chlorophyll a extracted. Treating sediments with acetone was found to yield higher concentrations of chlorophyll a, both for muddy and sandy sediments. Acetone was therefore found to be the best solvent for both sediment types, with 90% being the best strength for sandy and 80% the best for muddy sediments. These differences may be related to differences in the structure of the algal communities. Six hours of extraction was found to be sufficient, since after a six hour period the extraction efficiency did not improve.
- Does microphytobenthos resuspension influence phytoplankton in shallow systems? A comparison through a Fourier series analysisPublication . Brito, Ana; Fernandes, Teresa; Newton, Alice; Facca, Chiara; Tett, PaulShallow coastal lagoons, especially the ones with clear waters and lighted substrata, are likely to have large microphytobenthos (MPB) communities. MPB is an important component of these systems, representing up to 99% of the chlorophyll concentration when compared to phytoplankton. It is therefore expected that MPB resuspension play a key role in the dynamics of phytoplankton due to the tide and wind action. Water samples were collected twice per month inside and outside Ria Formosa lagoon (Portugal), for nutrients and chlorophyll a (chl a). Sediment samples were also collected for MPB chl a. Chl a was also analysed in water and sediment samples from Venice lagoon (Italy), at least once per month. A truncated Fourier series was fitted to the data to investigate the seasonal and high-frequency components of the time-series. In the Ria Formosa, the best significant fit for MPB was obtained considering the sum of 26 wave-pairs (sin and cosine), which explained 31% of the variability. The seasonal cycle (1e3 waves) explained approximately 5% of the total variability. Within-day variability which includes spatial heterogeneity explained 61% of the variability. The best fit for phytoplankton inside Ria Formosa was obtained considering the sum of 23 wave-pairs. Outside the lagoon the best fit was obtained using only the sum of 16 wave-pairs. For both cases, the sum of waves explained more than 64% of the variability and the seasonal cycle explained more than 31% of the variability. It is expected that primary producers in the water column have a strong seasonal factor due to the direct effect of the solar cycle, which is the case of other clear waters. In the Venice lagoon, which is microtidal, the best fit for MPB was obtained using 10 wave-pairs. However, the best fit for phytoplankton was obtained with only 3 wave-pairs, indicating the importance of the seasonal cycle. Significant relationships were found between phytoplankton inside and outside the Ria Formosa, as well as between microphytobenthos and phytoplankton in the lagoons of Venice and Ria Formosa. These results suggest the influence of MPB resuspension in the phytoplankton community of shallow coastal lagoons and the importance of phytoplankton exportation to the coastal zone.
- How will shallow coastal lagoons respond to climate change? A modelling investigationPublication . Brito, Ana; Newton, Alice; Tett, Paul; Fernandes, TeresaCoastal shallow lagoons are important ecosystems in terms of their high ecological relevance. They act as buffers of the landesea interface, providing valuable ecosystem services such as nutrient recycling, decomposition of organic matter and removal of pollutants. Lagoons are regions of restricted exchange, subject to anthropogenic pressures that result in problems such as eutrophication. Because they are shallow, submerged primary producers play a prominent role in lagoon system metabolism. Furthermore,coastal lagoons are particularly vulnerable to global climate change and may act as ‘sentinel systems’. Sea level rise already threatens to overwhelm some lagoons, such as Venice and Moroccan lagoons. Recent observations and studies have shown that a global climate change, especially the warming of the climate system and the sea level rise is unequivocal (IPCC, 2007). Therefore, the dCSTTMPB model, which deals with nitrogen and chlorophyll concentrations in the water column and within the sediments, was used to explore a range of scenarios that aimed at representing these changes. The tendency of the light limitation due to the sea level rise is the potential degradation of the microphytobenthos community. This reduction would lead to stronger nitrogen fluxes from pore water to the water column, increasing significantly the nitrogen concentrations. No increase in the phytoplankton community was found because it is mainly influenced by the resuspension of microphytobenthos. High nitrogen concentration may be the first indication of an eutrophication event. Nonetheless, nitrogen may be exported to the sea and cause problems in the adjacent coastal waters. The increase in temperature did not lead to significant differences.
- Sediment and water nutrients and microalgae in a coastal shallow lagoon, Ria Formosa (Portugal): Implications for the Water Framework DirectivePublication . Brito, Ana; Newton, Alice; Tett, Paul; Fernandes, TeresaCoastal shallow lagoons are considered to be highly important systems, which have specific biogeochemical cycles and characteristics. The assessment of sediment–water interfaces is essential to understand nutrient dynamics and to evaluate the vulnerability to eutrophication, especially in regions of restricted water exchange (RRE), such as the Ria Formosa, which have natural conditions for the accumulation of nutrients. Water samples were collected during the years of 2006 and 2007–08 for nutrients, chlorophyll a and dissolved oxygen. Sediment samples were also collected for pore water nutrients and microphytobenthic chlorophyll a. Measurements of temperature, salinity and photosynthetic active radiation were also taken. The lagoon salinity is affected by occasional strong rainfall events. From comparison with previous work, a decrease in the nitrogen concentration in the water column can be observed, which may indicate an improvement of the water quality. Pore water nutrient concentrations were significantly larger than in the water column. Sediment–water exchanges are considered to be the most important processes in nutrient dynamics of the lagoon. Benthic microalgal biomass was also large compared with that of the phytoplankton. It represents about 99% of the total microalgal chlorophyll biomass of the system. The lagoon also contains (discontinuous) meadows of intertidal seagrass, but we did not study these. Due to the importance of sediments, the standard monitoring plans required by the Water Framework Directive may fail to track changes in the nutrient conditions and the microalgal responses to them.
- Temporal and spatial variability of microphytobenthos in a shallow lagoon: Ria Formosa (Portugal)Publication . Brito, Ana; Newton, Alice; Tett, Paul; Fernandes, TeresaMicrophytobenthos (MPB) are an important, yet highly variable, component of productive shallow systems and intertidal areas. Samples were collected from Ria Formosa, Portugal, to assess the temporal and spatial variability of microphytobenthic chlorophyll in two types of intertidal sediment: mud and sand. Chlorophyll pigments were measured spectrophotometrically after freeze-drying and extraction into an acetone–water mixture. Time-series of superficial chlorophyll measured at two sites during 2006–2007 showed no obvious seasonal peaks. A truncated Fourier series was fitted to the time-series data. Seasonality was very weak: only 5% of the estimated total variance could be explained by annual cycle components up to 3 year 1, while 25% was explained by waves with periods from 14 to 91 days. The residual error of the Fourier series was partitioned into within-day variance (61%) and other components (9%). The within-day variation was made up of roughly equal contributions from: (i) variability associated with sampling within-sites; and (ii) differences between-sites. There were no significant correlations between MPB chlorophyll and tidal range, wind speed, solar irradiance, water temperature and salinity and water nutrient concentrations. Sediment type was once more confirmed to be a key factor to MPB spatial variability. These results are discussed in relation to processes controlling the distribution of benthic microalgae in Ria Formosa, and their implications considered in relation to on-going work to understand and model the role of microphytobenthos in eutrophication in such water bodies.
- The role of microphytobenthos on shallow coastal lagoons: a modelling approachPublication . Brito, Ana; Newton, Alice; Fernandes, Teresa; Tett, PaulRia Formosa is a Region of Restricted Exchange given its limited connection to coastal water circulation. Furthermore, it is subject to several anthropogenic activities that can lead to an increase in nutrients and potentially to eutrophication. Previous studies have shown the importance of the benthic compartment, specifically the microphytobenthos (MPB) in this shallow coastal lagoon. The dCSTT– MPB model [new version of the dynamic Comprehensive Studies Task Team (dCSTT) model] here described couples the benthic and pelagic compartments. Due to the shallowness of the system, the benthic microalgae are one of the most important primary producers of the system. Preliminary results of the model show a large biomass of benthic microalgae, which strongly influences the pelagic chlorophyll concentration by resuspension. However,algae concentrations in the water column are relatively small due to the high flushing rate of the lagoon. The MPB community is mainly supported by nutrients in the pore water. A sensitivity analysis (SA) has revealed that the factors associated with the benthic compartment were the most important and sensitive to changes. Porosity, benthic chlorophyll recycling, loss of MPB due to grazing and the yield of microphytobenthic chlorophyll from nitrogen were some of the most sensitive parameters, as well as the ones associated with decay of particulate organic nitrogen. The development of our dCSTT–MPB model has itself provided insights into benthic function.
- The yield of microphytobenthic chlorophyll from nutrients: enriched experiments in microcosmsPublication . Brito, Ana; Newton, Alice; Tett, Paul; Icely, John; Fernandes, TeresaThe yield of phytoplankton chlorophyll from nitrogen is a useful parameter in the study of eutrophication of coastal waters, providing the link between chlorophyll formation and nutrient consumption. This parameter has been estimated for microphytobenthos from Ria Formosa lagoon on the south coast of Portugal with a series of three experiments conducted in May and September 2007. Six sediment cores were collected from Ria Formosa in acrylic cylinders and cork stoppers were placed in the bottom. Water samples were collected in large containers and prepared (filtered and enriched) to be pumped to the top of the sediment inside the acrylic cylinders, which are the microcosms of the experiments. Microcosms were isolated to avoid water exchanges and placed in large tanks full of water to maintain temperature stability. Experiments were run for 5 to 9 days in a continuous diluted nutrient enriched system. One of the experiments was carried out with half the microcosms in the dark to evaluate the nutrient fluxes between the sediment and the water column, when no algal growth is expected. Nutrient fluxes of muddy sediments, during the two last days of the experiment in the dark were estimated to be 0.1015 μmol cm−2d−1 for nitrogen, −0.0015 μmol cm−2d−1 for phosphorus, and 0.1395 μmol cm−2d−1 for silicon. The yield of chlorophyll was determined to be between 3.65 and 4.11 μg chl (μmol N)−1 from nitrogen and between 4.03 and 4.77 μg chl (μmol Si)−1 for silicon in muddy sediments. It was not possible to determine the yield from phosphorus. These values are higher than for phytoplankton which may be due to physiological reasons, or due to the presence of a smaller fraction of microheterotrophs, which would divert nutrients. For sand, smaller yields were obtained but it is likely that this was mainly due to the fact that the values of the sediment fluxes used were the ones obtained for mud, which are higher than they should be for sand. This extra input of nutrients is considered to be taken up by algae, thereby decreasing the value of the yield. Therefore, reliable chlorophyll yield estimates for sand could not be obtained.
- Understanding the importance of sediments to water ouality in coastal shallow lagoonsPublication . Brito, Ana; Newton, Alice; Tett, Paul; Fernandes, TeresaThe undesirable enrichment of water by nutrients may be a problem, especially in areas with restricted exchange with the sea. The tidal regime flushes the system and contributes for the removal of phytoplankton, favouring phytobenthos as the target of enhanced nutrients. Water samples were collected during the years of 2006 and 2007-08 for nutrients, chlorophyll a and dissolved oxygen. Sediment sample s were also collected for pore water nutrients and benthic chlorophyll a. From comparison with previous work, a decrease in the nitrogen concentration in the water column can be pointed out, which may indicate an improvement of the water quality. Pore water DAIN represents approximately 75% of the total DAIN of the whole lagoon. Benthic chlorophyll a concentrations were much larger than in the water column, representing around 99% of the total chlorophyll existent in the lagoon. Benthic microalgae play a relevant role in this system and therefore standard monitoring programs of the WFD, which do not consider this component, may fail to track nutrient-driven changes in primary producers. Dissolved oxygen concentration could be near critical levels during the summer (early in the morning), especially in the inner channels.