Ratio-dependent response of a temperate Australian estuarine system to sustained nitrogen loading

Abstract

Classical resource- and the less studied ratio-dependent models of predator-prey relationships provide divergent predictions as to the sustained ecological effects of bottom-up forcing. While resource-dependent models, which consider only instantaneous prey density in modelling predator responses, predict community responses that are dependent on the number of trophic levels in a system, ratio-dependent models, which consider the number of prey per consumer, predict proportional increase in each level irrespective of chain length. The two models are only subtly different for systems with two or three trophic levels but in the case of four trophic levels, predict opposite effects of enrichment on primary producers. Despite the poor discriminatory power of tests of the models in systems with two or three trophic levels, field tests in estuarine and marine systems with four trophic levels have been notably absent. Sampling of phytoplankton, macroinvertebrates, invertebrate-feeding fishes, piscivorous fishes in Kooloonbung Creek, Hastings River estuary, eastern Australia, subject to over 20 years of sewage discharge, revealed increased abundances in all four trophic levels at the disturbed location relative to control sites. Increased abundance of phytoplankton at the disturbed site was counter to the predictions of resource-dependent models, which posit a reduction in the first trophic level in response to enrichment. By contrast, the increase in abundance of this first trophic level and the proportionality of increases in abundances of each of the four trophic groups to nitrogen loading provided strong support for ratio dependency. This first evidence of ratio dependence in an estuarine system with four trophic levels not only demonstrates the applicability of ecological theory which seeks to simplify the complexity of systems, but has implications for management. Although large nutrient inputs frequently induce mortality of invertebrates and fish, we have shown that smaller inputs may in fact enhance biomass of all trophic levels.

Oscillating trophic control induces community reorganization in a marine ecosystem

“…2005; Halpern et al. 2006) or bottom–up control (Ware & Thompson 2005; Bishop et al. 2006; Frederiksen et al.…”

Foraging distribution of Pacific harbor seals (Phoca vitulina richardii) in a highly impacted estuary

“…In addition, we calculated the distance between all areas of SFB and the locations of major outfalls and wastewater treatment dischargers in and around SFB. Sewage input has been linked with higher primary productivity in some estuarine areas (Surge and Lohmann 2002), and although large nutrient inputs have been linked with mortality of invertebrates and fish, smaller inputs may enhance biomass of all trophic levels (Bishop et al 2006). Data on the outfall locations came from the United States Environmental Protection Agency's National Pollution Discharge Elimination System permitting program, with each location representing the discharge point of a discrete conveyance such as a pipe or man-made ditch (National Pollution Discharge Elimination System 2001).…”

“…We developed a conceptual model of which environmental variables were most likely to influence spatial distribution of seals in our study area, a large coastal estuary subject to high levels of human use. Our hypothesis was that habitat use by seals would be higher in areas near haul-out sites, in areas of high prey abundance, and in areas of greater depth and higher bottom relief (where prey may be concentrated); and lower in areas of the estuary close to consistent sources of anthropogenic disturbance (i.e., areas with predictably high levels of human or boat activity), except commercial or recreational fishing areas, because these latter activities target areas of high fish abundance, or outfall locations, which could increase biomass of surrounding waters via moderate nutrient enrichment (Bishop et al 2006).…”

Relationships between estuarine modification and leaf litter decomposition vary with latitude

“…However, the influence of nutrient enrichment on organic matter decomposition extends beyond the supply and traits of detritus, to the decomposer community. Nutrient enrichment can stimulate the productivity of detritivorous invertebrates (Bishop et al, 2006;Cross et al, 2006; but see Pérez et al, 2013 for a counter-example) and…”