Isotopic niche size variability in an ecosystem engineer along a disturbance gradient in a South African lagoon

摘要

A key challenge for natural resource management is how to detect effects of environmental stress on individuals and populations before declines in abundance occur. Variability in carbon and nitrogen isotope composition (delta C-13 and delta N-15) among consumers can provide information on the population tmphic niche and how it may change in response to environmental stress. We measured delta C-13 and delta N-15 values in primary producers and in an ecosystem engineer, the bioturbating sandprawn Kraussillichfrus kraussi, in Langebaan Lagoon, South Africa, along a human disturbance gradient. Diet partitioning mixing models were coupled with isotope niche analyses and individual body condition data to investigate shifts in resource utilisation and diet plasticity from minimally to highly disturbed sites. The delta N-15 values of seagrass, Zostera capensis indicated a nutrient gradient, with the highest delta N-15 values at highly disturbed sites indicating either anthmpogenic or marine nitrogen inputs. A decreasing delta N-15 signal with distance from human disturbance/mouth of lagoon was however not evident for sandprawns nor their presumed dietary sources (phytoplankton, microphytobenthos or sediment organic matter), likely because of faster isotope turnover time compared to seagrass and/or differential fractionation for sandprawns among the sites. Sandprawn isotope niche sizes varied among sites, with no trend along the disturbance gradient. The smallest niche coincided with uniform feeding on microphytobenthos according to mixing models. On an individual level, deviating isotope values from population means were correlated to better body condition, suggesting that a divergent feeding strategy is beneficial. Our results support a generalist feeding behavior of the sandprawns with no evidence of reduced physiological status at the site with most human disturbance.