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

The fungal factor : male spider mites prefer fungally killed females to healthy live ones

The two-spotted spider mite, Tetranychus urticae, is a serious pest in a wide variety of crops worldwide, and can negatively affect the production of strawberries. Males of T. urticae approach and guard quiescent deutonymph females to increase their chances of fathering offspring, because only the first mate results in fertilization. Male mating behavior is a key step in spider mite reproduction. This study investigated the behavior of T. urticae males towards females killed by entomopathogenic fungal species in two different taxons: Neozygites floridana (Division Entomophthoromycota, Class Neozygitomycetes, Order Neozygitales) and Beauveria bassiana s.s (Division Ascomycota, Class Sordariomycetes Order Hypocreales). Single males were exposed to one of the following choice situations (treatments), each with two different types of non-moving females present: (A) healthy quiescent deutonymph vs. T. urticae cadaver killed by N. floridana; (B) healthy quiescent deutonymph vs. T. urticae cadaver killed by B. bassiana; (C) T. urticae cadaver killed by N. floridana vs. T. urticae cadaver killed by B. bassiana; and (D) healthy quiescent deutonymph vs. healthy adult freeze-killed. The females were placed individually on two partly overlapping leaf discs. Male behavior was observed and analysed during two minutes every hour for 6 hours. An analog CCTV camera was used to record the experiment, and the second observation in treatments C and D were analysed using Ethovision® XT8 software to observe the distance moved by the males. Results showed that males visited leaf discs with cadavers killed by fungi significantly more often than discs with healthy quiescent females. Female cadavers killed by fungi were also more touched and guarded by males than the healthy quiescent female. When males were exposed to two disease-free females (Treatment D) touching and guarding towards the freeze-killed female was less commonly observed than towards quiescent female. Further, males moved a longer distance in the presence of two fungal cadavers (C) than in the presence of two disease-free females (D). The results confirm the earlier finding that males prefer females killed by N. floridana and demonstrate that males exhibit similar behavior towards B. bassiana cadavers. This behavior could compromise the mating goal (chances of fathering offspring) and in addition promote the spread of entomopathogens. Understanding this phenomenon could be important to improve biological control of spider mites, and more research is needed to investigate the mechanism behind the attraction of T. urticae males to fungally killed female.M-ECO

Resultaten van stookolieslachtoffer-onderzoek langs de Vlaamse kust tijdens de winter 1992-93

With weakly counts of a permanent beach-traject (Oostende-Nieuwpoort: 16.5 kms), five monthly counts of the whole Flemish coastline and some additional counts, 850 km of beach has been surveyed for beached birds. Another 180 birds were collected from four rehabilitation centres situated along the Flemish coast. The winter 1992-93 started with only small numbers of beached birds ( Podiceps cristatus, Red-throated Diver Gavia stellata, Great Skua Stercorarius skua. Weakly counts of a permanent beach stretch give the best approach (n = 34) of the total number of species recorded (n=43). In species numbers, Ducks, Gulls and Waders are most important. In descending order, the Guillemot, Herring Gull, Kittiwake, Oystercatcher, Razorbill and Black-headed Gull are most abundant. The total number of beached birds along the Flemish coast in the period October 1992 -April 1993 is estimated at 1500-2000. This is 500 birds more than in the winter 1991-92. Based on the monthly counts, no significant differences between subtrajects could be demonstrated. Oil-rates were extremely low this winter (18%). High values in the period 1962-1975 were followed by a short decrease, and a new increase in the eighties. After peak-values in 1987, 1989 and 1990 oil-rates of seabirds and Guillemots decrease gradually unti11993. Why this low value has to be put into perspective, is discussed. Other mortality causes, found on the sampled birds, include entanglement in ropes and nets (Gannets!), stringing up of birds -deliberately or not -, swallowing of fishhooks (Gulls) and a variety of fractures and injuries, probably due to collisions with fast vessels or other objects and due to hunting. Finally the methods for beached bird surveys, as used this winter, are evaluated and same notes on the origin of beached birds on our beaches are made

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

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