Comparison of light trap designs for assessments of positively phototactic invertebrate and larval assemblages in urbanised coastal systems

Abstract

The construction of seawalls is increasing the extent of artificial infrastructure along coastlines globally. Such development compromises substrate complexity of the natural environment which is crucial for major life functions of larvae (invertebrates and fish). Understanding the physical and biological implications of how marine assemblages function within such transformed environments is imperative in view of the role of early life-stages in regulating the population dynamics of coastal species and the structure of coastal ecosystems. Appropriate sampling gear must be ensured for accurate and comparable data collection of plankton forms across various impacted environments, as in high wave-dynamic natural sites versus in sheltered, urbanised areas. Validating the sampling equipment is also warranted when assessing the cost-effectiveness of gear intended for deployment at wave-dynamic locations. This study evaluated the efficiency of two light trap designs (quatrefoil and cylindrical) in assessing photo-positive larval assemblages along the southeast coast of South Africa. The results showed no significant differences in species diversity, abundance or evenness between the two trap designs and among sites. Significant differences were, however, observed in species richness between the trap designs and across sites, and in overall species composition across sites. These differences were primarily attributed to the presence of juveniles or adults of taxa in the orders Cumacea and Amphipoda. Our findings suggest that either light trap design could be deployed at both urban and natural coastal ecosystems to assess the positively phototactic plankton assemblage from the early life stages. This methodological validation is critical for accurately refining scientific data collection and for meticulously monitoring complex life-stages of coastal resources that require management and habitat rehabilitation.