Organic enrichment or toxicity? A comparison of the effects of kelp and crude oil in sediments on the colonization and growth of benthic infauna

Abstract

The response of a benthic invertebrate community to two sources of sedimentary organic material was compared in a field experiment. The meiofauna and macrofauna colonizing and growing in sediment amended with varying concentrations (0,0.1,1 and 5%) and combinations of dry kelp debris and Isla Vista Seep petroleum were enumerated after 35 days at a subtidal site in the Santa Barbara Channel. Most groups of meiofauna responded similarly, having peak abundances at low nominal concentrations (0.1 % of one or both constituents), although kelp had generally a greater stimulatory effect than did oil. The distribution of life history stages of harpacticoid copepods indicated that proportions of females that were gravid and the ratios of nauplii to females increased with kelp added, but that the juvenile/nauplii ratios decreased with increased amounts of kelp, suggesting that survival of juveniles with increasing enrichment was determining patterns of harpacticoid distribution. Numbers of species of harpacticoid copepods exhibited a similar pattern to that of total numbers of harpacticoid copepods. Hierarchical clustering of the 16 treatments according to their Bray-Curtis similarity indices produced two main groups of harpacticoid copepods: a moderate oil ( x ̄ = 1.8% ) — low kelp ( x ̄ = 0.7% ) group and a moderate oil ( x ̄ = 1.8% ) — high kelp ( x ̄ = 4% ) group. For the macrofauna, as nominal oil concentrations increased peak responses in numbers of individuals at particular concentrations of oil shifted to higher concentrations of kelp. A hierarchical clustering of the macrofauna indicated two major groups of treatments: a low oil ( x ̄ = 0.3% ) — low kelp ( x ̄ = 0.3% ) group and a high oil ( x ̄ = 1.8% ) — high kelp ( x ̄ = 2.8% kelp ) group. Results therefore indicate that meiofauna respond clearly to one form of sedimentary organic matter, kelp, but not nearly as strongly to oil. Both forms of sedimentary organic matter were important to the numbers of individuals and species of macrofauna settling and growing in sediments. The numbers of nematodes and macrofaunal species other than Capitella spp. were correlated with sediment Eh, indicating that the toxicity of reduced by-products of bacterial metabolism, e.g., H 2S, may have an effect on some groups, partly explaining reduced numbers in treatments with higher amounts of added organic material. The data presented here are largely consistent with a view that the response of the benthic populations to crude oil in sediments are similar to those expected from other sources of organic matter.