Silica cycling in a freshwater tidal marsh

Silica plays a major role in eutrophication of coastal waters around the world. Mechanisms controlling the production and fate of silica in estuarine systems are far from understood. Major indications have been reported that intertidal areas may be an important reservoir of silica in estuarine systems. This project intends to clarify the role of a freshwater marsh in the silica cycle within the Schelde estuary. Different silica pools in the marsh (vegetation, sediment, pore-water, groundwater and surface water) are quantified on a two-monthly basis in different vegetation types. Dissolved Si, taken up by plants, is stored as amorphous biogenic silica, and is unavailable to the estuarine ecosystem until these plants decompose. Although the monitoring has not yet been carried out over the intended full year period, Phragmites australis and Urtica dioica could already be identified as the major vegetation sinks for dissolved silica in the marsh. Biogenic silica in surface sediments in the marsh increased from winter to summer. In spring and summer, the marsh becomes a sink for BSi, as diatoms and decomposing material are imported into the marsh. Mass-balances carried out may-june 2002 confirmed this theory. BSi was netto imported into the marsh. In contrary, it was observed that dissolved Si was netto exported from the marsh. The marsh seems to act as a reactor, transforming imported BSi to DSi, and thus makes this silica again available to the estuarine ecosystem. In the future, mass-balances will be carried out in all four seasons, to further clarify this theory. Interactions between the different silica pools will be studied by decomposition and dissolution experiments, both in situ and ex situ. In the end, these major goals will allow to construct an integrated view of the role of freshwater tidal marshes in the silica cycle within an estuarine system, by focusing on retention and processing of silica within the marsh

Possible effects of climate change on estuarine nutrient fluxes: a case study in the highly nutrified Schelde estuary (Belgium, The Netherlands)

Global change models predict effects of climate change on hydrological regimes at the continental scale in Europe. The aim of this study was to gain a better understanding of the possible effect of changing external forcing conditions on the functioning of estuarine ecosystems. In densely populated areas, anthropogenic nutrient enrichment and consequent alteration of nutrient biogeochemical cycles have already had a big impact on these ecosystems. The average yearly discharge of the upper Schelde estuary increased nearly threefold over the period 1996–2000, from 28 m3 s-1 in 1996 to 73 m3 s-1 in 2000. The continuously rising discharge conditions over the five-year period were used as a reference situation for possible future effects of climate on ecological functioning through increase of discharge. At high discharges, nutrient (NH4+, NO3-, dissolved silica and PO43-) concentrations in the tidal fresh- and brackish water showed a decrease of up to 50% while total discharged nutrient loadings increased up to 100%. Opposite effects of increasing discharge on NH4+, NO3- and dissolved silica concentrations in summer and winter, resulted in the flattening out of seasonal cycles for these nutrients. Under high discharge conditions, silica uptake by diatom communities was lowered. Dissolved silica loadings to the coastal area increased concurrently with total silica loadings upstream. Salt intrusion to the marine parts of the estuary decreased. This resulted in a downstream shift of the salinity gradient, with lower salinity observed near the mouth. As a result, TDIN, NO3- and dissolved silica concentrations doubled at the mouth of the estuary

Potential effects of global change on estuarine nutrient fluxes

One of the major worldwide problems in densely populated estuarine areas is the eutrophication of coastal waters. Studies, both observational and theoretical, addressing the issue of material fluxes to coastal zones under changing external forcing conditions, have a critical international importance. In contrast to N and P, the silica concentration in estuaries is hardly influenced by human pollution. Increased N-concentrations can lead to succession of diatom communities to phytoplankton communities with less favorable properties. Global change models predict effects of climate change on hydrological regimes at the continental scale in Europe. Schelde freshwater discharges could increase up to 28 %. Strongly increasing freshwater discharges over the period 1996-2000 in the upper Schelde estuary could be an example for future changes in estuarine and coastal response to excessive nutrient loading due to human impact on the global climate. Effects in the upper estuarine areas were totally different to effects at the mouth of the estuary. In winter, when discharge increase was highest, dilution resulted in lower concentrations of NH4+, PO43- and total nitrogen in the upper and brackish parts of the estuary. Nitrate and oxygen concentrations increased. Significant regressions were observed between trends and discharge regime. In summer, when discharge increases were not as high as in winter, no dilution was observed. Moreover, lower residence times in the freshwater due to higher discharges, have a negative effect on water quality in the brackish estuary in summer, as more unprocessed NH4+ is transported downstream, which results in very low oxygen conditions. In summer, high discharges result in the complete flushing of entire diatom communities in the freshwater reach of the estuary, which resulted in much higher dissolved Si concentrations. Total discharged loads of nitrogen, phosphorus and silica increased spectacularly over the study period. Nitrate and silica concentrations in the coastal waters, the two main actors in coastal eutrophication, were significantly correlated to total yearly discharges observed upstream. Effective measurements against non-point pollution and insight in the role of intertidal areas in regulating non-point nutrient fluxes become more important than ever in the light of increasing discharge

Evolution of water quality in the freshwater Zeeschelde (96-00): a reason for optimism? (poster)

The evolution of water quality in the freshwater part of the Zeeschelde was monitored since 1996. Until now, most research in the Schelde estuary has focused on the marine and brackish part of the Zeeschelde. Often, concentration trends are used to evaluate the success or absence of success of pollution control measures. The total discharge of nutrients to the brackish and marine part of the estuary from the freshwater upper estuary is a function of both the concentration of these nutrients in the freshwater and the total volume of water discharged. It is important to realize that a change in nutrient concentration does not automatically implicate a change in nutrient loading. Assessing the success of restoration programs by concentration trends only is therefore not sufficient. Discharge influence on nutrient and oxygen concentration was compared seasonally between winter and summer period. It is clearly shown that observed amelioration of water quality must almost certainly be attributed to the strongly increasing discharges during the same period. If we measure water quality by nutrient loads exported to the lower estuary, the same increasing discharge results in heavily increasing loads of nutrients

The ecological functioning of the Scheldt estuary: towards integration of research

The Scheldt Estuary is confronted with a loss of functionality, mostly if ecological functions are considere. The system capacity of purifying water is weakened. The ecological infrastructure is scattered. Flood waves gain strength. It is a scientific challenge to quantify to what degree tidal wetlands can support restoration of the ecological functioning of the estuary. It is illustrated that an integrated multidisciplinary approach is a satisfying strategy to obtain adequate system knowledge so that the complex role of wetlands can be understood. The results of OMES, an integrated research program are presented for this purpose. Mass balances indicated that tidal wetlands aerate the water column, remove nitrogen from the overlying water and regenerate dissolved silica. Sedimentation takes place, but soil formation only happens in the most elevated parts. The interactions with the wetland vegetation were targeted at different levels. On the level of individual plants, nutrient removal from the root zone was studied. This resulted in a diagenetic model. On species level (in casu Phragmites australis), a model was developed that allows predicting growth under different factors. On plant community level, a model was constructed that shows how development of tidal marsh vegetation is mainly controlled by local management, flooding frequency and the salt gradient. The coupling of these models formed a marsh submodel unit that can be incorporated in an ecological model covering the whole estuary

Anthropogenic impact on amorphous silica pools in temperate soils

Human land use changes perturb biogeochemical silica (Si) cycling in terrestrial ecosystems. This directly affects Si mobilisation and Si storage and influences Si export from the continents, although the magnitude of the impact is unknown. A major reason for our lack of understanding is that very little information exists on how land use affects amorphous silica (ASi) storage in soils. We have quantified and compared total alkali-extracted (PSi<sub>a</sub>) and easily soluble (PSi<sub>e</sub>) Si pools at four sites along a gradient of anthropogenic disturbance in southern Sweden. Land use clearly affects ASi pools and their distribution. Total PSi</sub>a</sub> and PSi<sub>e</sub> for a continuous forested site at Siggaboda Nature Reserve (66 900 ± 22 800 kg SiO<sub>2</sub> ha<sup>−1</sup> and 952 ± 16 kg SiO<sub>2</sub> ha<sup>−1</sup>) are significantly higher than disturbed land use types from the Råshult Culture Reserve including arable land (28 800 ± 7200 kg SiO<sub>2</sub> ha<sup>−1</sup> and 239 ± 91 kg SiO<sub>2</sub> ha<sup>−1</sup>), pasture sites (27 300 ± 5980 kg SiO<sub>2</sub> ha<sup>−1</sup> and 370 ± 129 kg SiO<sub>2</sub> ha<sup>−1</sup>) and grazed forest (23 600 ± 6370 kg SiO<sub>2</sub> ha<sup>−1</sup> and 346 ± 123 kg SiO<sub>2</sub> ha<sup>−1</sup>). Vertical PSi<sub>a</sub> and PSi<sub>e</sub> profiles show significant (<i>p</i> < 0.05) variation among the sites. These differences in size and distribution are interpreted as the long-term effect of reduced ASi replenishment, as well as changes in ecosystem specific pedogenic processes and increased mobilisation of the PSi<sub>a</sub> in disturbed soils. We have also made a first, though rough, estimate of the magnitude of change in temperate continental ASi pools due to human disturbance. Assuming that our data are representative, we estimate that total ASi storage in soils has declined by ca. 10 % since the onset of agricultural development (3000 BCE). Recent agricultural expansion (after 1700 CE) may have resulted in an average additional export of 1.1 ± 0.8 Tmol Si yr<sup>−1</sup> from the soil reservoir to aquatic ecosystems. This is ca. 20 % to the global land-ocean Si flux carried by rivers. It is necessary to update this estimate in future studies, incorporating differences in pedology, geology and climatology over temperate regions, but data are currently not sufficient. Yet, our results emphasize the importance of human activities for Si cycling in soils and for the land-ocean Si flux

Is relative Si/Ca availability crucial to the performance of grassland ecosystems?

​Species composition of grasslands and pastures is an important control on biomass production and ecological functioning, with a significant role of grasses and legumes. A change in composition of legumes/grasses abundance and biomass ratio results in altered nutrient cycling and composition of higher trophic-level communities (e.g., grazers). However, in addition to pasturing and fire effects, other parameters may also potentially affect grassland composition. Grasses are known as silicon (Si) accumulators and legumes as calcium (Ca) accumulators. We propose a new testable hypothesis, and a conceptual model, on the role of Si/Ca availability in controlling legume/grass dominance/competition in grassland systems. Based on available literature, we argue that Si/Ca availability is an important trigger for shifts in abundance of both plant families. The differential uptake of Si and Ca by legumes and grasses affects grassland biogeochemistry and microbial (fungal) biomass. In addition, altered litter stoichiometry, through impact of Ca and Si uptake on N, C, and P turnover, affects the decomposition processes