Ecohydrological Functioning of a Tropical Monsoon and a Temperate River Floodplain

Abstract

Floodplains are important ecosystems that play a vital role in storing water during times of high river discharge. Unfortunately, many floodplains have become disconnected from their rivers. To properly manage rivers and their floodplains, we need to better understand the complex interactions between water, soil, plants, and animals. Studying rivers and floodplains that are minimally impacted by human intervention can provide valuable insights. In this thesis, we compare two relatively undisturbed rivers, the Narew in Poland and the Songkhram in Thailand, to explore similarities and differences between floodplains in temperate and tropical climates. We examine vegetation zoning and spatial patterns in water quality, composition, and nutrient availability, which are related to flood characteristics like duration, depth, and frequency. We found that both floodplains act as nutrient sinks, storing rather than releasing nutrients to the river. Along the river, tall vegetation made up of bamboo (in the Songkhram) or reeds, grasses, and sedges (in the Narew) filters water and supports high productivity. As vegetation becomes less productive further from the river, the influence of river water decreases. This zoning creates a high level of biodiversity that's closely tied to river and floodplain dynamics. While nitrogen is the main nutrient that regulates vegetation in both areas, nutrient supply from flood water and soil varies by location and vegetation type. The Narew's water discharge is relatively constant year-round, whereas the Songkhram has highly variable discharges, with high peaks during post-monsoon flooding and very low discharges during the dry season. To develop effective floodplain conservation, management, and restoration plans, we need to consider climate, river discharge patterns, nutrient loads, local conditions, vegetation zones, and how these factors interact. Empirical studies like this one, though labor-intensive, can provide a better understanding of the system as a whole and enable a targeted approach to conservation and restoration efforts in different climate zones.