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

Kelp forest POM during upwelling and downwelling conditions: using stable isotopes to differentiate between detritus and phytoplankton

David C. Dyer1,*, Mike J. Butler2, Albertus J. Smit3,4, Robert J. Anderson1,5, John J. Bolton1

1Biological Sciences Department, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa
2Environmental Isotope Laboratory, iThemba LABS, Private Bag 11, WITS, Johannesburg, 2050, South Africa
3Biodiversity and Conservation Biology Programme, University of the Western Cape, Private Bag X17, Bellville, 7535, South Africa
4South African Environmental Observation Network, Elwandle Coastal Node, Port Elizabeth, 6031, South Africa
5Branch: Fisheries, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Roggebaai, 8012, South Africa
*Corresponding author:

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


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Cite this article as: Dyer DC, Butler MJ, Smit AJ, Anderson RJ, Bolton JJ (2019) Kelp forest POM during upwelling and downwelling conditions: using stable isotopes to differentiate between detritus and phytoplankton. Mar Ecol Prog Ser 619:17-34. https://doi.org/10.3354/meps12941

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