Aquatic ecosystem responses to fire and flood size in the Okavango Delta: observations from the seasonal floodplains

Abstract

The frequency of fires in the Okavango Delta seasonal floodplains peaked at an intermediate frequency of flooding. Floodplains are commonly burnt every 3-5 years. This study showed fundamental changes in ecosystem properties due to burning. A burnt seasonal floodplain in the aquatic phase had oxygen levels well above saturation, 100-200%, while the levels in the un-burnt control site were below saturation and, at night, could decline to 10-40% saturation. The total phosphorous and total nitrogen concentrations were similar on both floodplains but considerably enriched relative to inflowing water, due to nutrient release from the flooded soil-sediment and animal droppings. Zooplankton biomass was very high in both systems although the abundance of fish fry was ten times higher on the un-burnt floodplain. In a low flood year the un-burnt floodplain water had high nutrient levels, primary production, methane emission, and subsequent uptake of methane in biota, as well as a high zooplankton biomass. The very high flood the following year showed the opposite with much lower production at all levels owing primarily to greater dilution of nutrients. The abundance of fish, however, was much higher during the high flood year. Macrophytes and litter provide direct shelter for fish fry but also promote low oxygen levels when decaying. Large flooded areas result in high fish production by removing obstacles related to congestion. This interplay between hydroperiod and fire may be crucial for the maintenance of high biological productivity both in the aquatic and terrestrial phases in a very nutrient poor wetland landscape. Understanding these interactions is crucial for optimal management.

Seasonal and spatial hydrological variability drives aquatic biodiversity in a flood‐pulsed, sub‐tropical wetland

“…, 2002; Bunn, Davies & Winning, 2003; Lindholm et al. , 2007; Ramberg et al. , 2010), influences water chemistry (Mackay et al.…”

“…Furthermore, flood events connecting isolated areas are reported to homogenise community composition (Thomaz et al, 2006), whereas here the period of greatest connection with the arrival of the flood pulse increased heterogeneity in water chemistry and invertebrate richness and community composition. The arrival of a flood pulse precipitates a burst of primary productivity fuelling the aquatic food web (Bayley, 1995;Høberg et al, 2002;Bunn, Davies & Winning, 2003;Lindholm et al, 2007;Ramberg et al, 2010), influences water chemistry (Mackay et al, 2011), detritus processing and the presence of vertebrate predators (Mosepele et al, 2009), and therefore has both a direct and indirect influence on the invertebrate communities. The abundance and diversity of invertebrates were highest on the rising and peak flood, generally, but not exclusively, at low HP class sites.…”

Tropical wetlands: seasonal hydrologic pulsing, carbon sequestration, and methane emissions

“…On the seasonally flooded floodplains there is a positive correlation between mean frequency of flooding and the frequency of fires up to a level of seven flooding years and three fires (Ramberg et al 2009) after which the fire frequency drops. These trends are readily explained by the increase of macrophyte primary production in the floodplains caused by higher flooding frequency and the resulting higher fuel load as a determinant for fire frequency (Heinl et al 2006); fire is less likely where flood frequency is greater than 7 years in fifteen because the increased wetness reduces the possibility of burning.…”

Algal richness and life‐history strategies are influenced by hydrology and phosphorus in two major subtropical wetlands

“…The species-energy theory (Wright, 1983;Hubbell, 2001) posits that energy supply, that is, resource quantity, controls and limits species richness. In the Okavango, P limits algal growth, especially during high floods (Ramberg et al, 2010), or is co-limiting with nitrogen (N; Mackay et al, 2011). In the Everglades, natural P supply is extremely low, and so N concentrations are rarely regulatory (Noe, Childers & Jones, 2001).…”