Linking allometric macrobenthic processes to hypoxia using the Peters mass balance model

Abstract

Eutrophication alters coastal aquatic food web structure and function through effects of organic enrichment and hypoxia. Macrobenthic communities provide good ecological indicators of impacts on ecosystem function induced by eutrophication. However, effective resource management requires validation of ecological indicators through mechanistic links to specific stressors of concern. Organic enrichment along with subsequent hypoxia typically engenders depauperate macrobenthic communities consisting of small short-lived surface-dwelling organisms. Metabolic ecology theory offers great promise for understanding the role of body size within the context of the trophic dynamics of aquatic ecosystems. Body size is a fundamental ecological trait because it underpins vital rates, which can be scaled up to the ecosystem level through allometric laws. Thus, body size-dependent mechanisms can potentially be used to link macrobenthic indicators to ecosystem consequences of organic enrichment and hypoxia. The Peters mass balance model (PMBM) [Peters, R.H., 1983. The Ecological Implications of Body Size. Cambridge University Press, New York, 329 pp.] provides a platform for projecting stress-induced changes in the distribution of biomass among discrete size classes of organisms. In the PMBM, changes in the distribution of biomass ensue over time from differences between gains due to ingestion and losses due to egestion, respiration, and mortality. Production of biomass results when ingestion exceeds combined losses. In the present study, the PMBM was modified to envisage an inhibitory effect of hypoxia on ingestion by: (1) using unique size-specific mortality coefficients in order to realize target inverted biomass-size distributions during model initialization; (2) linking ingestion rate to the hyperbolic relationship between oxygen consumption rate (OCR) and ambient dissolved oxygen (DO); (3) hypothesizing how oxygen regulation ability follows an allometric scaling rule relative to the supply of DO. If food is unlimited, the most straightforward hypothetical connection between the oxygen consumption rate and ingestion assumes a direct proportional relationship under oxygen limitation. Although small organisms can attain much higher mass-specific rates of oxygen consumption than large ones, previous limited research on oxygen consumption relative to oxygen supply implies that large organisms regulate oxygen intake better than small individuals as ambient DO declines. Counterintuitively, linked ingestion deficits would be accordingly higher for small organisms than large organisms under hypoxia. In pilot simulations incorporating hypothetical ingestion deficits due to oxygen limitation within a modified PMBM, contrasting scenarios emulated continuous DO and water temperature conditions experienced at two shallow subtidal sites for 32 d in summer. Resulting ingestion deficits reduced total biomass and favored large size classes. Effects were stronger under the more hypoxic scenario: total biomass was lower and the biomass-size distribution was more uneven among the size classes. Differences were even more evident in terms of an integrative macrobenthic process indicator, the normalized biomass-size spectrum (NBSS). A realistic hypoxia mass balance model (HMBM) must incorporate various allometric effects that are not necessarily expressed in the same direction relative to body size. A useful resource management tool for anticipating effects of environmental stress on aquatic ecosystem function could be developed from the quantification of trophic transfer potential within a refined PMBM.