Assessment of Secondary Production and Efficiency of Different Mesh Sizes to Study Benthic Communities Associated to a Zostera marina Meadow

Abstract

Aims: The problem addressed in this paper, was to reduce the number of sieves in order to propose appropriate methodologies for the estimations of abundance, secondary production, community composition and structure of benthic fauna associated to a Zostera marina meadow. Therefore we are aiming to calculate in an efficient way, secondary production of benthic communities in coastal areas associated to a Zostera marina meadow and examine the retention efficiency of different mesh sizes. Study Design: Bimonthly five randomly chosen cores of 15 cm of diameter and 18 cm length (area=176.7 cm²), were collected. Place and Duration of Study: The samples were taken during one year period from June 2008 to June 2009 in a Zostera marina meadow at Punta Banda estuary, a shallow coastal lagoon located near Ensenada, Baja California, Mexico (31º43-46’ N and 116°37-40’W). Methodology: Samples were immediately fixed with 7% buffered formalin, later, samples were washed through a nested series of sieves (5.6, 4.0, 2.8, 2.0, 1.4, 1.0, 0.70, 0.50 mm) stacked in descending order of size. In the laboratory fauna collected from each sieve was sorted and taxa classified using a stereoscopic microscope, organisms were counted (abundance, density) and preserved in 70% ethanol. Wet weight was measured using a Sartorius precision balance with a resolution of 0.001 g, for each group and each sieve. A general method to obtain Ash-free-dry weight (AFDW) of each core, which is the value of the dry weight minus the weight of the ash. Results: A total of 14760 organisms were collected and classified in 11 phyla; we found densities ranging from 17457 to 33600 ind. m–2. Benthic fauna was separated using eight different sieves (5.6 mm to 0.5 mm). The observed mean annual secondary production was 14.8 g (AFDW) m2 y-1. The projected mean using equivalences was 12.2 g (AFDW) m2 y-1. A student t-test showed no significant differences between this two means. We suggest using the weight equivalences proposed by Crisp to find reliable AFDW estimates. Conclusion: Results indicate that using only two sieves of 1.0 and 0.5 mm, allows an appropriate estimation of abundance of individuals in small size classes. Crisp equivalences procedure to assess secondary production reduces the amount of time and effort needed to obtain AFDW assessments without causing bias in the interpretation of community composition and structure.