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Research Document - 2004/045

Estimating zooplankton biomass from dry weights of groups of individual organisms

By Head, E.J.H, Harris, L.R.

Abstract

Mesozooplankton biomass is measured for samples collected in the AZMP (Atlantic Zone Monitoring Program) either as wet weight, after draining the entire sample, or as dry weight for one half of the sample, after splitting, filtering on to pre-weighed filters and drying at 60oC for 24 h. Mesozooplankton samples are often contaminated with large amounts of phytoplankton whose biomass will be included in the mesozooplankton sample wet and dry weights determined using the AZMP protocol. Between 1998 and 2001, another set of mesozooplankton samples (Head and Harris samples) were collected during AZMP cruises on the Scotian Shelf in the 0-100 m depth range. These were analysed using a different protocol. Counts and dry weights were determined for mesozooplankton, by picking individual organisms and placing them into a series of groups, according to stage and species, genus, or some broader taxonomic group. The groups were dried (60oC, 1-4 days) and average individual dry weights were determined. The total dry weight biomass was calculated by summing the dry weights of the organisms in each of the component groups. This method excludes the contribution of phytoplankton. Amongst the groups for which dry weights were measured were each of the developmental stages of Calanus finmarchicus, C. glacialis and C. hyperboreus and groups of mixed stages of Oithona spp. and Pseudocalanus spp.. Average individual dry weights were lower in fall than in spring for all of the C. finmarchicus stages and Oithona spp.. The same trend was apparent for the other Calanus species, although in the fall only a few stages were encountered and these at only a few stations. Seasonal average individual dry weights were generally similar in eastern and western regions of the Scotian Shelf. Seasonal average individual dry weights and abundance estimates were used to calculate the sum of the dry weights for only these five mesozooplankton species/genera for the Head and Harris samples. In spring these accounted for over 60% of the total mesozooplankton in most cases and often over 70%. The same individual dry weight averages were applied to the abundance data for AZMP samples from the Scotian Shelf. The summed dry weights of these five species/genera were sometimes less than and sometimes greater than the measured dry weights. Most of the measurements were within a factor of two of the calculated weights, however. The discrepancies may be partly due to the splitting, sub-sampling and counting procedures used in the AZMP protocol. Combining individual dry weight and abundance data allows estimation of the contributions of these important groups to the total mesozooplankton biomass. Average individual dry weights of late state Calanus spp. on the Labrador and Newfoundland Shelves were similar to Scotian Shelf spring values in May and early June but ca. 30% higher in late June and July. Other groups also showed some differences. Thus, average seasonal individual dry weights values derived from one region should not be used for other regions without validation.

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