PARENT SESSION
Organized Oral Session 6: Development of landscape heterogeneity at multiple scales in wetlands
Organizer(s): B Warner and A van der Valk
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 516 A, Level 5, Palais des congrès de Montréal

The role of biota in landscape-level heterogeneity in the Okavango Delta, Botswana.

Ellery, William^*,1, McCarthy, Terence ², ¹ School of Environmental Sciences, Durban, KwaZulu-Natal, South Africa² School of Geosciences, Johannesburg, Gauteng, South Africa

ABSTRACT- Landscape level heterogeneity in the Okavango Delta is a product of historical changes in flow and spatial heterogeneity in elevation and surface water, groundwater and soil chemistry. All of these factors are controlled largely by biota, which: (1) focuses clastic bed-load sediment to in-channel areas in the upper fan, thereby causing rapid channel aggradation and instability that leads to radical changes in flow patterns over timescales of decades to centuries; (2) promotes deposition of suspended clastic sediment and removal of dissolved nutrients in the upper fan, which result in the creation of striking gradients in nutrient supply away from primary channels; and (3) promotes focussing of detrimental solutes to islands in the lower fan, which leads to local-scale salinisation of groundwater and soils as well as to the creation of topographic relief. These processes have dramatic consequences for local scale heterogeneity such that community distribution varies in relation to: (1) successional stage following drying and wetting after channel avulsion; (2) variation in nutrient availability away from primary channels as a consequence of preferential uptake of nutrients at the head of the fan; and (3) variation in groundwater and soil chemistry on islands as a result of the creation of hydraulic gradients from wetland to island, which is accompanied by the selective uptake and exclusion of dissolved solutes by plants, leading to local salinisation of groundwater and terrestrial soils. The importance of these processes for overall ecosystem structure and functioning is also fundamental in that they affect local and regional gradients, and they ensure that despite the high potential for salinisation of surface water (potential evapotranspiration greatly exceeds rainfall in every month of the year such that 98% of the total water supply in the system is lost to the atmosphere), dissolved solid load in surface water only doubles from 40-50ppm at the head of the fan to 100ppm at the toe. Thus, landscape level heterogeneity as well as system-level characteristics are fundamentally controlled by biological processes.

Key words: okavango delta, landscape heterogeneity, ecosystem engineers