Changes in tropical seagrass food webs across a gradient of nutrient enrichment: How might a resource subsidy maintain simplification?
Tewfik, Alexander*,1, 4, Rasmussen , Joseph2, McCann, Kevin 3, 1 McGill University, Montreal, Quebec, Canada4 University of South Florida, Tampa, Florida, USA2 University of Lethbridge, Lethbridge, Alberta, Canada3 University of Guelph, Guelph, Ontario, Canada
ABSTRACT- Overgrazing and subsequent simplification of marine macrophyte dominated systems has often been linked to predator and competition release of urchins due to over-fishing. However, the contribution of nutrient enrichment to the progression and persistence of this phenomenon should also be considered given that eutrophication often results in significant changes in food web structure. In an effort to understand the contribution of nutrient enrichment to changes in tropical seagrass food webs we examined a number of beds across the Caribbean along a gradient of increasing nutrient enrichment. The patterns of increasing consumer density, dominated by generalist urchins (>98%), and decreasing consumer functional group diversity corresponded well to increasing enrichment and loss of autochthonous seagrass detritus. The persistence of high urchin densities (> 66000/ha) was thought possible through a subsidy from opportunistic producers (i.e. phytoplankton), and associated detritus, that are known to benefit directly from elevated input of allochthonous nutrients. A more fine scaled isotopic (d13C, d15N) assessment of food web structure was conducted at both low and high nutrient enriched sites in the Dominican Republic to further explore the proposed subsidy. The general pattern at the enriched site indicated significant shifts in d13C of non-generalist consumers (i.e. specialist grazers, sub-surface deposit feeders) and sediment organic material (SOM) to lighter carbon sources as well as shifts in the contribution of POM (phytoplankton and particulate organic material) (7% to 44%) and seagrass detritus (57% to 24%) to the SOM mixture. The availability of alternative, allochthonous resources to generalist urchins, in conjunction with other flexible life history traits, could potentially allow for the persistence of simplified seagrass communities where high densities of urchins continue to suppress benthic macrophyte production through grazing and the recruitment of benthic consumers through intra-guild predation.
Key words: eutrophication, community, simplification, subsidy
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