Does epibenthic structure negatively affect meiofauna?

Abstract

The epibenthic harpacticoid copepod Parastenhelia megarostrum attains very high population densities in troughs of persistent sediment ripples on open intertidal sand banks in Pauatahanui Inlet, New Zealand. In closely adjacent eelgrass Zoslera capricorni Aschers beds, individuals of this species are only infrequently recorded from subcanopy sediments. Epibenthic structure in the form of flexible artificial seagrass plants was emplaced in the ripple field at two different times (June, September). Field experiments were undertaken to test the premise that presence of such structure would, by the creation of turbulent water flows around plants, lead to a progressive alteration in the microtopographic profile of the sediment leading to reductions in abundance of P. megarostrum. Moreover, these experiments were designed to test the effects of imposed overhead structure not only on this surface-dwelling copepod but also other components of the primarily infaunal meiobenthos. The experimental design consisted of two location treatments (inside and outside mimic grass plots) and two density levels of mimic plants within plots (30, 60 plants · 0.5 m −2), sampled at eight intervals representing time in days (D) since implantation of mimic plants (D1,2,4,6,8,12,16,31). Core samples were taken from within experimental plots, with replicates taken outside acting as undisturbed controls. The imposition of artificial epibenthic structure resulted in the rapid breakdown of ripple integrity, marked changes in profile and a net accumulation of sediment over the experimental period in both plant density treatments. In both 30- and 60-plant density plots, there were highly significant interactions between experimental period and location for P. megarostrum, with decreases in abundance inside plots. No equivalent interactions or main effects on abundance of the interstitial harpacticoid Paraleptastacus sp. or nematodes were revealed. Variance analyses indicated that abundance of P. megarostrum is less dependent on gross changes in the sediment surface than to the presence of overhead cover and that high densities in sediment depressions are a function of active aggregation rather than passive accumulation. Modification of sediment surface profile and mechanical disturbance by the sweeping action of seagrass blades clearly has a significant impact on epibenthic meiofauna but not on infaunal species.