Temporal and spatial variability in stable isotope ratios of SPM link to local hydrography and longer term SPM averages suggest heavy dependence of mussels on nearshore production
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Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ13C and δ15 N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration.
KeywordsPhytoplankton Macroalgae Suspend Particulate Matter Agulhas Current Suspend Particulate Matter Sample
Dr. S. Davis and Dr. R. Kalin at the EERC, Queens University Belfast provided invaluable assistance, guidance and instruction as well as the use of their facilities. Dr. M. Villet and Dr. F. Porri, Rhodes University, provided statistical help and direction. Isotope analysis was done both through the EERC, Queens University Belfast and the stable light isotope unit, University of Cape Town. All of this help is gratefully acknowledged.
- Allanson BR, Read GHL (1995) Further comment on the response of Eastern Cape Province estuaries in South Africa. S Afr J Aquat Sci 21:56–70Google Scholar
- Brown AC, McLachlan A (1990) Ecology of sandy shores. Elsevier, AmsterdamGoogle Scholar
- Caldeira RMA, Russell P, Amorim A (2001) Evidence of an unproductive coastal front in Baía d’Abra, an embayment on the south east of Madeira Island, Portugal. Bull Mar Sci 69:1057–1072Google Scholar
- Fry B, Sherr EB (1984) δ13C measurements as indicators of carbon flow in marine and freshwater ecosystems. Contrib Mar Sci 27:13–17Google Scholar
- Hunter M, Price P (1992) Playing chutes and ladders: heterogeneity and the relative roles of bottom-up and top-down forces in natural communities. Ecology 73:724–732Google Scholar
- McQuaid CD (1985a) Seasonal variation in the ash-free calorific value of nine intertidal algae. Bot Mar XXVIII:545–548Google Scholar
- McQuaid CD (1985b) Seasonal variation in biomass and zonation of nine intertidal algae in relation to changes in radiation, sea temperature and tidal regime. Bot Mar XXVIII:539–544Google Scholar
- Miyake Y, Wada E (1967) The abundance ratio of 15N/14N in marine environments. Rec Oceanogr Work Jpn 9:32–53Google Scholar
- Newell R, Shumway S, Cucci T, Selvin R (1989) The effects of natural seston particle size and type of feeding rates, feeding selectivity and food resource availability for the mussel Mytilus edulis Linneaus, 1758 at bottom culture sites in Maine. J Shellfish Res 8:187–196Google Scholar
- Vorwerk PD (2007) A preliminary examination of selected biological links between four eastern Cape estuaries and the inshore marine environment. PhD Thesis, Rhodes University, p 268Google Scholar