INFLUENCE OF HIPPO AND CATTLE DUNG ON THE NUTRIENT LEVELS, ALGAL DIVERSITY, COMPOSITION, BIOMASS AND ECOSYSTEM METABOLISM IN AQUATIC ECOSYSTEMS: A MESOCOSM APPROACH

Abstract

Large mammalian herbivores (LMH) are major agents of the movement of terrestrial organic matter and nutrients into aquatic ecosystems, which can alter the structure and functioning of aquatic ecosystems. Populations of native large mammalian herbivores have been lost or displaced by livestock in many regions of the world, and this may result in changes in the functioning of the aquatic ecosystem due to significant differences in the quality of their dung. A replicated mesocosm experiment was conducted for six weeks (42 days) at the bank of Amalo river to study the influence of large mammalian herbivores (hippo and cattle) dung input on nutrient concentration, algal biomass, composition and diversity, and flume-scale ecosystem metabolism. The treatments included: H100 (no cattle dung, 100% hippo dung), H80 (20% cattle dung, 80% hippo dung), H60 (40% cattle dung, 60% hippo dung), H40 (60% cattle dung, 40% hippo dung), H20 (80% cattle dung, 20% hippo dung), and H0 (100% cattle dung, no hippo dung). These treatments were conducted in triplicate. Nutrients were analyzed by standard colourimetric methods, chlorophyll-a was extracted using methanol and measured spectrophotometrically, total suspended solids (TSS) and ash-free dry mass (AFDM) were determined gravimetrically and algae were counted under a microscope using the counter rafter cell and flume-scale ecosystem metabolism was measured in-situ using miniDOT loggers; where dissolved oxygen and the water temperature was recorded every one minute for the six weeks. Study results showed that cattle and hippo dung inputs have contrasting effects on aquatic ecosystem function; treatment with 100% cattle dung (H0) released higher amounts of limiting nutrients, exhibited higher algal biomass, higher algal abundance and diversity and supported higher rates of gross primary production (GPP) relative to 100% hippo dung (H100). Hippo dung, which has slower mineralization rates and larger particle sizes, increased water clarity relative to cattle dung. Differences in stoichiometry (C: N:P ratio) of major elements and faecal particle sizes between cattle and hippo dung were the likely drivers of decomposition and releasing rates of nutrients, which in turn influenced ecosystem processes differently. The study suggests that large mammalian herbivores (LMH) play a significant role in the movement of organic matter and nutrients from terrestrial to aquatic ecosystems. The loss or displacement of native LMH populations by livestock in various regions of the world can lead to changes in the functioning of aquatic ecosystems due to differences in the quality of their dung.