Si-WEBS (home page)

Si-WEBS
Natural and anthropogenic modifications of the Si cycle along the land-ocean continuum: Worldwide Ecological, Biogeochemical and Socio-economical consequences

Project objectives

The general objective of Si-WEBS is to integrate the Si biogeochemical cycle into a human perspective, by evaluating the impacts of natural and anthropogenic perturbations of the coastal Si cycle onto (a) the ecology of coastal ecosystems, (b) the socio-economics of the coastal zone, and (c) the biogeochemistry of the global Si and C cycles. This general objective can be tied to key action 3 of the work programme for Environment and Sustainable Development. The project objectives fit to following key actions of the European Commission:

Sustainable Marine Ecosystems - Global Change, Climate and Biodiversity - Sustainable management and quality of water

Top-to-bottom coupling between the Si and C biogeochemical cycles

We will combine retrospective analyses, process based experimental work, and integrative modelling studies in order to improve the parameterisation of the Si:C coupling in biogeochemical of river systems, the coastal zone and the global ocean. The work will focus on achieving a better understanding, in the various biogeochemical provinces, of the processes which control (a) the contribution of diatoms to the total primary production, (b) the coupling between Si, C and N during silicification, and (c) the competition between recycling and preservation of Si and C, both in the water column and at the sediment-water interface. A particular emphasis will be placed on the role of the pelagic-benthic coupling in both ecosystem functioning (bottom-up perspective) and the relative preservation of Si and C in the sediments (top-down perspective).

Silica transformations along the LOC

We will integrate the results of the individual studies mentioned above in order to improve our capability to predictively simulate Si transformations along the LOC. We will build a coupled river/coastal zone model, so as to understand Si retention and transfer on the basis of "first principles". DSi inputs in an Ocean General Circulation Model (OGCM) model will be regionalised in order to determine the time scale of the ocean response to altered Si cycle in the coastal zone. The river and coastal zone models will first be connected conceptually but a realistic parameterisation of such a coupling requires additional experimental work. Particular emphasis of this experimental work will be placed on (a) quantifying the relative importance of DSi inputs from the well-known weathering of silicate rock minerals and the unknown phytoliths (link to the terrestrial cycle) and groundwaters, and (b) studying the fate of freshwater micro-organisms when mixed with marine waters and vice versa. The latter part includes quantifying the dissolution kinetics of various types of particulate silica, including phytoliths and freshwater diatoms (more or less enriched in lithogenic elements such as Al, Mg, Fe and K) as they enter salinity gradients, and marine diatoms with variable degrees of silicification or degradation (including the stimulating role of bacteria). Si retention alng the LOC will be explored in the form of BSi but also lithogenic silica (LSi). The largely unknown role of reverse weathering reactions will be extensively studied.

Ecological consequences of altered Si cycle along the LOC

Case studies have documented long-term declining Si:N and Si:P ratios and concomitant development of harmful algal blooms in coastal waters. However, studies are scattered throughout the literature. We will take advantage of existing databases and build upon the existing case studies reported in the literature to develop a web based database in order to analyse long-term trends in nutrient concentrations and fluxes at the land-sea interface. Data trend analysis will be complemented by the use of our coupled river/coastal zone model to perform scenarios of altered DSi delivery and coastal zone response in various climatic regions, with either nutrient enrichment, river manipulation, climate change, or any combination of these stressors. This combination of modelling and retrospective analysis should assess the role of Si in the functioning of coastal ecosystems, with important implications for their sustainable management.

Socio-economical consequences of altered Si cycle along the LOC

Consequences of DSi induced modifications of phytoplankton composition will be explored up to the highest trophic levels and the socio-economics aspects evaluated in terms of the impact of Si on the international fish sector (prices, production, competition) as well as on marine biological diversity. Given the ecological and socio-economical consequences of an altered Si cycle along the LOC, we will provide end-users with Si based tools that can be used for coastal ecosystem management in a perspective of sustainable development. These tools will include (a) Si:N and Si:P nutrient ratios as Early Warning Indicators (EWI) of dysfunction in the coastal zone caused by silica deficiency; (b) DSi and PSi (BSi + LSi) as core parameters in Coastal Monitoring Programs; (c) the coupled model to run scenarios and study the effects of various watershed management policies, including conceptual silica fertilisation experiments to alleviate DSi limitation and study its effects on aluminium toxicity and coastal food webs.

Biogeochemical consequences of altered Si cycle along the LOC

The perturbations of coastal food webs with changing nutrient ratios also have implications for the biogeochemical cycling of C in the coastal ocean, through modifications of diatom sinking losses. Strong links will be developed with the Continental Margin Task Team (CMTT) of the International Geosphere/Biosphere Programme (IGBP), to integrate the role of Si and diatoms in their study of the role of continental margins in the global C cycle. Conversely, the CMTT database will be combined with our study of Si retention as based on first principles, to search for the missing Si sink in the coastal ocean. In the open ocean, we will exploit the Si database collected within the framework of the EU-SINOPS project, to better define the coupling between the Si and C cycles in the global ocean. Besides simulations with the Hamburg OGCM, this analysis will also help us to test the role of the silicate pump as an important control on oceanic new and export production with important implications for the rain ratio among biogenic particles and atmospheric CO2 concentrations.

Such a comprehensive and interdisciplinary study will lead to major breakthroughs in our understanding of the processes controlling the Si cycle during land-ocean interactions, and the major impacts that these Si transformations have on the ecology, the biogeochemistry, and the socio-economy of the coastal ocean as well as on the global Si and C cycles.

info@pangaea.de