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ABSTRACT. Environmental variables, and summer and winter particulate concentrations in the water column of a kelp bed in an upwelling region on the western Cape coast of South Africa, were measured daily to assess short-term fluctuations. There were significant differences between the summer and winter upwelling index, water temperature and wave height. Summer part~culate carbon and nitrogen concentrations of 533 11g 1-' and 66 pg 1-' respectively were lower than winter levels of 764 pg C 1-' and 79 pg N 1-'. However, summer upwelling events did not reduce particulate carbon and nitrogen to the very low levels that might be expected from previous studies of chlorophyll concentrations in the kelp bed. Large waves resulted in C:N ratios that were significantly hlgher in winter than in summer @ < 0.01), and during continuous winter downwelling, a large proportion of the particulate material in the water column was of macrophyte rather than phytoplankton origin. In summer the carbon and nitrogen load was most highly correlated with temperature, while wind and wave size were the most important factors affecting particulate concentrations in winter. Mean particulate organic concentrations were 380 big C I-' and 43 pg N I-' and these compare weU with concentrations of 301 k~g C 1-' and 4 1 pg N 1-' estimated from previous primary production studies. The estimated carbon demands of Choromytilus meridjonalis and Aulacomya ater, 2 mussels that commonly occur in kelp beds on the west coast, could be met in winter and summer by the particulate organic component in the kelp bed water column. Particulate nitrogen resources probably meet most of the nitrogen requirements of the 2 mussels.

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Algal pigment concentrations in the water colun~n of a southern Benguela kelp bed were measured d d y during summer and w n t e r using High Performance Liquid Chromatography (HPLC) and spectrophotometnc methods. Chlorophyll a concentrations estimated by HPLC were significantly lower than those estimated by the trichromatic equations of Jeffrey & Humphrey (1975; Biochem. Physiol. Pflanz. 167: 191-194), although differences were relatively small. There was no significant difference between chlorophyll a concentrabons estimated by HPLC and the spectrophotometric equation of Lorenzen (1967; Limnol. Oceanogr. 12: 243-246). Mean HPLC chlorophyll a concentrations were 1 64 pg 1-l in summer and 1.78 ,ug 1-' in ulnter. Previous spectrophotometric estimates of kelp bed chlorophyll a concentrabons were generally comparable to those determined by HPLC. In summer, 5 d pulses of southeasterly winds caused upwelhng of cold nutr~ent-rich water into the kelp bed, which resulted in phytoplankton blooms. Time series analysis indicated that HPLC chlorophyll a concentrations were positively correlated with the upwelling index of 5 d previously, swell height of the same day and sea temperature of the previous day. In winter, winds were mainly northwesterly, sea temperatures were stable, and chlorophyll a concentrations were negatively correlated with swell height of 2 d previously. Phaeopigment concentrations determined by the method of Lorenzen (1967) reached 7 04 pg 1-' and were significantly higher than phaeophytin a concentrations determined by HPLC, which rarely exceeded 2 pg 1-' Chlorophyllide a occurred in 57 % of summer samples and 43 % of winter samples, and was more prevalent than phaeophytin a. ChlorophyUlde a and chlorophyll a concentrabons of the same day were significantly correlated in both summer and winter. Fucoxanthn concentrations reached 5 35 ,ug I-' in winter and 2.04 pg 1-' in summer, and daily winter fucoxanthln concentrations were highly correlated with chlorophyll a concentrations. Fucoxanthin and chlorophyll c, pigments characteristic of kelps and diatoms, occurred in > 90 % of wlnter samples and in < 35 % of summer samples. The kelp bed particulate resource in winter was therefore qualitatively different from the summer particulate resource.