MEPS 619:17-34 (2019)  -  DOI: https://doi.org/10.3354/meps12941

ABSTRACT: Kelp forests are highly productive temperate rocky reef ecosystems, often dominated by filter-feeding organisms which consume particulate organic matter (POM) and phytoplankton suspended in the water column. Variability in POM composition was hypothesized to be related to upwelling processes in and around a typical kelp forest on the west coast of South Africa. Ecological variables including chl a, C:N ratio, particulate organic carbon:chl a ratio, and total suspended solids were used to characterise the composition and determine the dynamics of the POM along 2 transects (alongshore and offshore) originating within the kelp forest. The natural variability in POM composition of upwelling and downwelling periods was used to isolate phytoplankton and kelp stable isotope (δ¹³C and δ¹⁵N) end-member values. Stable isotope analysis was employed to estimate the contribution of kelp-derived detritus to the POM present in the water column, including contribution estimates for Ecklonia maxima and Laminaria pallida. Under upwelling conditions, stable isotope analysis confirmed the dominance (>70%) of kelp detritus in POM samples, even at distances of 7.5 km offshore. Under downwelling conditions, however, phytoplankton was dominant (>60%) along both transects. This study therefore highlights the importance of coastal processes such as upwelling for controlling the composition of POM in kelp forests, as well as illustrating how the natural variability in POM composition created by upwelling processes can be used to gather POM end-member isotope values.

KEY WORDS: Particulate organic matter · Laminariales · Kelp-derived detritus · KDD · Ecklonia · Laminaria · Stable isotope analysis · SIA