The ecology of Ruppia drepanensis Tineo in a Mediterranean brackish marsh (Donana National Park, SW Spain) : a basis for the management of semiarid floodplain wetlands

One of the most important features of floodplain wetlands, both from an economical and a purely naturalistic point of view, is their high secondary production, with abundant invertebrates, fish and birds. This production is primarily based on the high productivity of aquatic macrophytes and their associated periphytic algae. Semi-arid floodplain wetlands have a very dynamic character, with yearly recolonization by the fauna and flora as a characteristic phenomenon. It is a consequence of the annual cycle of inundation and subsequent desiccation. The present study concentrates on the understanding of the dynamics of the aquatic vegetation as a research strategy for the management of such wetlands. Aims of the study were to identify potential causes for the decline of the submerged macrophyte populations, and to better understand the factors behind the interannual variation in their development.

The marsh of the Doñana National Park (SW Spain) was selected as a study area. The research concentrated on the growth, photosynthetic performance and reproduction of the dominant macrophyte species in the brackish area of the marsh, Ruppia drepanensis Tineo. Working hypotheses were generated from a conceptual model explaining the life cycle and biomass development of the submerged macrophyte populations. In this model, we consider the light climate as the main factor limiting the development of the submerged vegetation. High nutrient concentration is hypothesized as the main factor triggering the decline of the submerged macrophytes, either by a indirect shading effect due to increased growth of periphyton, or by a direct physiological effect on the plant development (toxicity of ammonia) and reproduction (delay of the flowering event caused by high nitrogen availability).

Light intensity and temperature strongly influenced growth and reproduction. The plants showed a strong capacity for photosynthetic acclimation to low light intensities. A weaker acclimation capacity was coupled with a high plasticity with respect to the temperature effect on photosynthesis.

Photoperiod and photosynthetic period did not influence growth, but affected reproduction. A longer photoperiod resulted in earlier flowering, and a longer photosynthetically active period resulted in more flowering and eventually higher seed production. Flowering was triggered by temperatures above 15 °C, and was strongly reduced at 30 °C. Although in all cases some plants were able to flower, the set of characteristics necessary for a successful reproduction can be defined as a long photosynthetic photoperiod and a range of temperatures above 15 °C but below 30 °C.

Relatively high nitrate concentrations in the water column resulted in postponed flowering both under high and low sediment nutrient concentrations. Under field conditions, this can result in a complete failure of the reproduction, as the wetlands dry up after a 3 to 4 months period suitable for vegetative growth.

High ammonia concentrations in the root zone resulted in a limitation of growth and a failure of the reproduction. Photosynthetic oxygen production was not affected, but respiration increased strongly. The effect was more pronounced at higher temperatures. We conclude that, under hypertrophic conditions, toxicity of the ammonia generated in the anaerobic sediment layer can severely limit the development of the submerged plant populations.

Low bicarbonate levels at the end of the season strongly restricted the photosynthetic production. Plant age also had a significant effect, with low photosynthetic production after the beginning of seed production. Together with the depletion of nutrients and with the effect of high temperatures on photosynthesis, these factors can explain the decline of the vegetation at the end of the season, sometimes before the actual drying up of the bigger wetlands.

Two years of field data confirmed the importance of wind-induced sediment resuspension and periphyton growth in influencing the light climate experienced by the submerged vegetation. Phytoplankton was always scarce in the areas were submerged vegetation was developing. Plant biomass increased exponentially in early spring (March), with steady biomass yields (up to 100 g afdw m ^-2) together with abundant flowering and fruiting in late spring (April-May). Interannual. variation was found to be very high, both concerning the abundance and the distribution of the submerged vegetation, mainly because of differences in rainfall which influenced the inundation cycle. Grazing by waterfowl can also account for this effect, as in dry years birds concentrate in the few wetlands still containing water.

For our conceptual model, we conclude that there is experimental evidence for direct physiological effects of high nitrogen concentrations on the decline of submerged macrophyte populations. This finding can complement the general hypothesis of a negative periphyton- mediated effect of high nutrient loads on the aquatic plants during the process of cultural eutrophication. Low light conditions have a strong, negative effect on the development of macrophyte populations. Macrophyte growth tends to improve the underwater light climate by stabilizing the sediment and reducing the attenuance caused by seston particles. Then, shading by periphyton, bicarbonate depletion and low nitrogen and phosphorus levels tend to be the main growth limiting factors. Flowering and seed production are thus quickly followed by plant senescence. Long-term survival of the seed bank and recolonization from seeds from neighbouring wetlands, the role of plant diversity in increasing the stability of the submerged vegetation meadows, the effect of grazing by herbivorous fauna, and the effect on light climate of sediment resuspension by carps, flamingoes and cattle trampling are proposed as elements worthwhile to study in the future.

Finally, some examples are provided for the use of the present conceptual model for the management of semiarid shallow wetlands.