Seaweed farming and land-use impacts on seagrass meadows in the region of Rote Island, Indonesia

Abstract

The international demand for macroalgae products has increased driving the expansion of seaweed farming to many coastal rural areas worldwide. Yet it is not fully understood what the impacts of this practice are on local habitats, including seagrass meadows, one of the most productive coastal ecosystems. Additionally, high nutrient inputs to coastal systems via direct freshwater run-off or groundwater can cause algal blooms with negative consequences to seagrasses and seaweed production. This study aims to detect the short term ecological impacts of seaweed farming on seagrass meadows, as well as the influence of nutrient inputs via groundwater on seaweed productivity in the near-shore aquatic system of Rote Island, Indonesia. Seaweed farming plots were set up over seagrass meadows in two locations on the island with assumed higher and lower anthropogenic pressure in Nemberala village and Tunggaoen beach, respectively. Seaweed production rates, seagrass indices, and environmental indicators were measured within the seaweed farm and in plots directly outside of the farm over two seaweed growth cycles. Seaweed production was positively affected by environmental changes during the rainy season in the region, including lower temperatures, higher water motion and higher nutrient concentrations. On the other hand, results show 15% reduction in seagrass shoot density, as well as reduced leaf length, plant biomass, and percentage cover under the seaweed farms. Nuisance algae were found attached or entangled in the seaweed fronds, especially in Nemberala, causing further light reduction under the farms. The impacts on seagrasses also led to diminution of Corg (22-23%) and nitrogen (22-29%) in surface sediment in the plots compared to unfarmed areas. δ15N of the suspended particulate matter of sampled groundwater from the beach seepages in both sites were high, 13.4‰ in Nemberala and 17.8‰ in Tunggaoen, indicating heavier sources of N either from sewage contamination or isotopic discrimination due to microbial processing of groundwater. During the rainy season, the N- and P-enriched water entering the system led to an increase in nutrient concentrations in Nemberala bay seawater, causing eutrophication.