Identification of kelp zoospores from in situ plankton samples

Studies of the link between planktonic and benthic stages of marine macroalgae have been hampered by the inability to identify planktonic stages from field samples. Here I describe a microphotometric method for identifying kelp zoospores from in situ plankton samples based on light-absorbance characteristics of zoospore plastids. Absorption spectra were obtained from plastids within Macrocystis pyrifera (L.) C. Agardh, Pterygophora californica Ruprecht, Eisenia arborea Areschoug and Laminaria farlowii Setchell zoospores released in the laboratory from adult specimens collected in the field; these are the dominant subtidal kelps in most southern California kelp forests. Absorption spectra were highly specific to each kelp species. Average absorbance decreased from P. californica to E. arborea to L. farlowii to M. pyrifera at most wavelengths between 400 and 750 nm. Discriminant-function analyses provided a significant discriminant, based on zoospore-plastid absorbance values at 25 different wavelengths, for identifying unknown zoospores among the four kelp species. Cross-validation techniques indicated that the discriminant rule could successfully identify 97.8% M. pyrifera, 96.4% P. californica and 69.7% L. farlowii zoospores. Most misidentified E. arborea zoospores were identified as L. farlowii, and vice-versa; when the relatively rare E. arborea was pooled with L. farlowii and treated as a single complex, the discriminant rule could successfully identify 98.2% M. pyrifera, 97.4% P. californica and 89.5% E. arborea/L. farlowii zoospores. Coupled with a protocol for isolating kelp zoospores from non-kelp cells, this discriminant rule was used to determine kelp zoospore abundances and species-compositions of: in situ plankton samples. This is the first description of a method for identifying macroalgal zoospores or spores from field samples.