Towards a global coastal typology for modelling nutrient fluxes at the land-ocean interface: an application to lagoons.

Cheryl M. Van Kempen⁽¹⁾, Goulven G. Laruelle⁽¹⁾, Hans H. Dürr⁽²⁾, Caroline P. Slomp⁽¹⁾, Hans Middelkoop⁽²⁾, Michel Meybeck⁽³⁾
(1) Department of Earth Sciences – Geochemistry, Faculty of Geosciences, Utrecht University, The Netherlands
(2) Department of Physical Geography, Faculty of Geosciences, Utrecht University, The Netherlands
(3) UMR Sisyphe, Université Paris VI, France

full text: cvkempen_Gnux2007.pdf (17 Mb)

Abstract

Nutrients, and in particular nitrogen (N) and phosphorus (P) play important roles in both terrestrial and aquatic ecosystems, as they are essential to sustain primary production. Sudden excessive increases in nutrient availability however also have the potential to disrupt ecosystem balance and its ability to provide ecosystem services. As a large part of the world population depends on coastal ecosystem services for their livelihoods, such as fishing and tourism, there is a particular interest to understand the biogeochemical cycles and to estimate nutrient levels of N and P in the coast world wide. To achieve this at the global scale, means for upscaling available data to uncovered areas are required. GIS allows a spatial typologic approach to coastal biogeochemical research, as will be shown here.

In the present thesis, the first steps are made towards the development of a geochemically relevant coastal segmentation which, in future work will be coupled to generic nutrient box-models. The ensemble of segmentation and models will allow estimating nutrient levels and retention in coastal systems anywhere on earth. Here, the first preliminary steps are set, by developing a coastal typology and lagoon nutrient model.

A coastal segmentation (at 0.5° — 0.5° resolution) is presented here, which is biogeochemically relevant and in which all segments were classified according to a filtering typology. The classes (I, II, III and 0) represent both the magnitude and processes driving nutrient filtering in estuarine, lagoon and fjord-like coasts and finally coasts where no relevant biogeochemical activity takes place in the proximal coastal ocean, but instead in the distal coastal ocean. Each of these types makes up 40, 10, 30 and 20% of the world’s coast respectively, and they receive 30, 7, 10 and 53% of the global annual runoff. The coastal segmentation was obtained by aggregating classes of a morphologic typology, also presented here. The 11 morphologic classes represent karst, fjord, fjärd, ria, macrotidal, estuarine, lagoon, big river, mangrove, arheic and glaciated sedimentary coasts.

To assess the complexity in terms of the mathematical representation of the nutrient fluxes the generic nutrient model can have, three site specific models are presented here for type II (lagoon coasts). They are applied to Thau, Venice and Szczecin lagoons and are simple, coupled, multi-element (N and P), multi-form (dissolved/particulate, organic/inorganic), process-based box models. All processes are represented by rate constants, and depend only on the source reservoir. The importance of exchange of water between the lagoon and the sea is assessed, as well as a sensitivity analysis, comparison of rate constants a simulation of Szczecin lagoon for the period 1980 to 1999 and finally an assessment of the effect of the N:P ratio for consumption on lagoon biogeochemistry. The results show 1) that exchange with the sea must be included in a lagoon model; 2) primary production and remineralisation cannot be represented well by first order kinetics alone and 3) the ratio at which N and P are consumed during photosynthesis affect DIP and primary production considerably.

Preliminary Global Coastal Typology

(for Nutrient filtering at the land-ocean interface)