Trophic responses to nutrient enrichment in a temperate seagrass food chain

Abstract

Simple ecological models that predict trophic responses to bottom-up forcing are valuable tools for ecosystem managers. Traditionally, theoretical ecologists have used resource-dependent functional responses to explain the modification of food chains exposed to bottom-up perturbations. These models predict alternating positive, negative and zero responses at each trophic level. More recently, ratio-dependent functional response models that predict proportional increases at each level have challenged this paradigm. The present study tested the predictions of the 2 hypotheses empirically by comparing the relative biomasses of 4 trophic levels of an estuarine seagrass food chain in relatively undisturbed, low-nutrient catchments and 'developed' catchments subjected to a prolonged period of nutrient enrichment. We found that nutrient-enriched sites had significantly greater biomass of both epiphytic algae and grazing invertebrates; however, the bottom-up forcing of nutrients was attenuated at higher trophic levels (occupied by juvenile and piscivorous fish), with no significant effect of catchment development. This disconnect in the upward cascade of energy may be due to a number of possible reasons including high levels of diversity and omnivory, trophic subsidy within the system or the strength or nature of perturbations. Although the predictions of both hypotheses failed to hold across all trophic groups, ratio dependence was prevalent at the lower levels of the food chain, which has implications for catchment management.

Identifying knowledge gaps in seagrass research and management: An Australian perspective

“…Increases in computing power has allowed for advances in statistical and spatial analyses and modeling, and have led to the new field of bioinformatics that enables the exploration of very large datasets produced by new technologies (Lucas et al, 2012;Procaccini et al, 2007). The synthesis of the large volumes of environmental and experimental data into models of habitat suitability, threat assessments, growth dynamics in relation to environment and climate, trophodynamics, and hydrodynamics will lead to greater predictive power of how seagrasses will respond to future scenarios of change and perturbation (e.g., Grech and Coles, 2010;Maxwell et al, 2015;Petus et al, 2014;Saunders et al, 2013;York et al, 2012). Understanding the complexities of the interactions affecting seagrass ecosystem and the growing human pressures that are impacting them will also require an interdisciplinary approach to investigation bringing together researchers from many different fields.…”

Positive responses of a seagrass ecosystem to experimental nutrient enrichment

“…After the sample had been taken, the dispensers were replaced and all fish were enumerated by species. Juvenile fish known to prey on grazing invertebrates were identified as per York et al (2012), then dried with paper towel and weighed. To sample the above-ground Zostera muelleri biomass (hereafter called Z. muelleri biomass), blade surface area and length, as well as epiphytes and epiphyte-grazers, a 50 cm long × 20 cm diameter steel cylinder with a 500 µm sieve at the top was randomly placed within each site 1, 3, 5, 7 and 9 mo after the commencement of experimental enrichment.…”

“…predation), become increasingly important in the resilience of seagrass ecosystems (Heck et al 2006, Verhoeven et al 2012. Resolving the relationship between nutrient loading and seagrass ecosystem health therefore requires an understanding of the responses of seagrass, epiphytes and grazers to sustained bottom-up forcing (York et al 2012), as well as impacts to higher order predators and top-down controls (Heck & Valentine 2007, Moksnes et al 2008.…”

Evaluation of the enrichment and amplification effect of pentachlorobenzene withlower bioconcentration in the food chain before and after modification

“…In this article, four species were selected: Chlamydomonas reinhardtii, Daphnia pulex, Danio rerio, and pelican, which can form a food chain in nature (C. reinhardtii → D. pulex → D. rerio → pelican). It is possible to reduce the concentration of pollutants in the human body by reducing the enrichment at each trophic level of the food chain [36].…”