High larval abundances in the Ria Formosa (Southern Portugal)-methodological or local effect?

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Abstract
Zooplankton abundance at a station in the Ria Formosa (Southern Portugal) was monitored every week or second week over 18 months at incoming tide, high tide, ebb tide and low tide. The water samples were filtered by 32μ mesh and examined under an inverted microscope. The proportion of planktonic larvae was high all year round. They formed 50–70% of the metazoic plankton organisms during winter and 80 to >90% during summer. An extrapolation implied that this corresponds to 40–80% of the biomass in summer and <20% in winter. On average, roughly three-quarters of the larvae were nauplius larvae; the remaining quarter were meroplanktonic stages, especially of gastropods, bivalves and polychaetes. Larval abundances varied independently of tide (high tide-low tide), but showed a significantly negative correlation with the tidal elevation (spring tide-neap tide) for total larval number and for most larval groups separately. It is outlined that next to a geographical component, these relatively high larval figures are the result of the mesh size used. When treated as a component of the mesozooplankton, larval number and biomass will always be greatly underestimated.

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