Biomass and Production Estimates for an Estuarine Meiobenthic Copepod, with an Instantaneous Assessment of Exploitation by Flatfish Predators

It has long been known that estuarine basins playa critical role as nursery areas and sheltered feeding habitats for the juveniles of a number of commercially important fish species. Yet within New Zealand the precise value of such ecosystems from this point of view remains largely unexplored. In his summation of a seminar on nutrient processing and biomass production in New Zealand estuaries, held at the Cawthron Institute in 1982, Knox (1983) concluded that most current data were fragmentary and took little account of the broader interactions between various components of the estuarine biota. Moreover, one of the most severe handicaps was the lack of even the most fundamental quantitative data on abundance, trophic interactions, and estimates of biomass and secondary production. Before any generalizations can be made about how typical New Zealand estuaries might be compared with their more intensively studied counterparts elsewhere in the world, there is an urgent need to make available, even in preliminary form, estimates of functional characteristics such as biomass, production, and food web relationships. The meiobenthic harpacticoid copepod Parastenhelia megarostrum Wells, Hicks and Coull, is the dominant epibenthic metazoan on Mana Bank in Pauatahanui Inlet, Porirua Harbour. This species is not only extremely abundant at this locality, but it also constitutes the principal item of prey for young post metamorph flatfish during their first half year of benthic life (see Hicks, 1984). These a-group fish are within a size range of about 8.0 to 35.0 mm standard length, and may be observed occupying the same realised niche in estuaries and shallow harbour flats throughout New Zealand. This note, which complements an earlier report (Hicks, 1984), provides a first assessment of biomass and secondary production of the P. megarostrum population in Pauatahanui Inlet, and evaluates the consequences of predatory removal by flatfish. Fortnightly sampling was undertaken from March 1981 to April 1982 from an intertidal fine sand bank (Mana Bank) in Pauatahanui Inlet, the eastern arm of Porirua Harbour (41 °06'5; 174°54'E). Details of sampling procedures and environmental characteristics are available in Hicks (1984). Preliminary estimates of biomass and production were obtained as follows. Dry weight values were predicted from body length / dry weight regressions presented in Goodman (1980) and Fleeger and Palmer (1982), but with allowance made for a body morphology appropriate to P. megarostrum (see Hicks and Coull, 1983, p.72). An adult female thus has a dry weight ≈ 2.30μg. A more conservative approach which accommodates some individuals within the juvenile (copepodite) size range, is to calculate average dry mass weights according to Faubel's (1982) definite size class method. This yields a generalized mass of 0.56μg. individual-1. Standing stock determinations using this latter value or that predicted from adult females, are here regarded as lower and upper estimates respectively. Biomass (B) was derived as the product of the mean annual population density (March 1981-March 1982 = 263 individuals. 10 cm -2) and individual dry weight (adult female = 2.30μg) as 0.605 grams ash free dry weight. m -2 yr -1 ,or assuming 40% of dry weight is organic carbon (e.g. Feller, 1982), 0.242 g C. m -2 yr -1. This upper value can be set against the lower one derived from a definite size class measurement (0.56μg. individual-1) of 0.147 gafdw. m-2 yr-1, or 0.059 g C. m-2 yr-1. Production to biomass (P / B) ratios of 9 have been widely used for meiofaunal organisms when information on the number of generations is not available (Heip et al., 1982). When the number of generations is known, P / B ratios have been shown to vary greatly between 2 and 26 per year (Gerlach, 1971; Heip, Herman and Coomans, 1982). Based on 14 months in situ population data for P. megarostrum, I concluded that up to 7 generations might be produced annually (Hicks, 1984), although these overlapped greatly, making precise cohort identification impossible. Accepting that the 5 major recruitment pulses evident in this population represent discrete and successive yet merging generations (Hicks, 1984, Fig.4), and assuming Waters' (1969) generalized ratio of 3 for cohort production to mean standing crop, this gives an