Are mangroves in arid environments isolated systems? Life-history and evidence of dietary contribution from inwelling in a mangrove-resident shrimp species

Abstract

The Arabian Gulf represents one of the more northerly extremes of mangrove distribution in the Indo-Pacific, and is populated only by Avicennia marina, due to its tolerance of high salinity and wide temperature extremes. Recent studies suggest that in the arid coastal environment of the western Arabian Gulf, export of carbon and nitrogen from mangroves to adjacent habitats may be limited, though it is not clear if this is due to low productivity or physical factors such as the lack of freshwater flow and the tidal regime. Although seagrass and macroalgal habitats are relatively much more dominant by area, with only small pockets of mangrove around the edges of embayments, it is not evident if inwelling from these habitats support mangrove fauna. Year-round sampling in mangroves at Al-Khor, Qatar, indicates that Palaemon khori, an endemic shrimp species, is strongly associated with mangroves throughout its post-settlement life cycle, from recruitment as small 9–10 mm juveniles through to mating and egg production. Rapid post-recruitment growth (k = 1.8, L∞ = 42 mm for females, k = 1.5, L∞ = 35 mm for male) means that most individuals reached adult size in the first few months after settlement, with reproduction occurring in the following spring. As might be expected from year-round residence in the mangrove, dual 13C and 15N isotope analysis indicated a strong contribution of mangroves to shrimp tissue growth (Mean and 95% confidence range, 37% and 27–48%), but with a weaker significant contribution from particulate organic matter (20% and 1–37%), mangrove epiphytes (16% and 2–33%) and seagrasses (9% and 0.2–18%). Other primary producers contribute the remaining 18% to shrimp nutrition but the 95% confidence ranges include zero, suggesting possibly non-significant roles in supporting the shrimp population. This dietary information supports the view that fauna resident within arid mangrove systems are mainly dependent on localised retention and cycling of nutrients with a smaller contribution from inwelling.