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detectable temperature effect. It is yet uncertain how simultaneous changes in light
and nutrient regimes with temperature increases would influence seaweed-grazer
relationships. But it seems that any climate-driven temperature effects on species-
specific seaweed-grazer interactions may not propagate linearly through the food
web, and depend, among others, on the body size, diversity, diet specificity, and
connectivity of members of the food web (Petchey et al. 2010 ; see also Chap. 19 by
Roleda and Hurd).
The seaweed-grazer effects of other variables subject to global change, such as
sedimentation or ocean pH, are poorly understood. Many coastal regions experi-
ence an increase in sedimentation, mostly due to coastal erosion and, in cold-
temperate and polar regions, the increased melting of glaciers (Bogen and Bønsnes
2003 ). Regions under strong sediment influence are characterized by lowered
seaweed and invertebrate density and diversity (Spurkland and Iken 2011 ). How
this translates into specific effects on seaweed-herbivore interactions is not yet
known. Similarly, it is known that increased ocean acidification reduces the abun-
dance and fitness of crustose coralline algae on coral reefs and may enhance fleshy
algal growth, but the specific impacts of pH on grazers and on seaweed-grazer
relationships are unknown (Kuffner et al. 2008 ).
8.7 Conclusions
Despite the many advances achieved in the broad field of grazer-seaweed
interactions, many if not most topics require more comprehensive investigation to
reach a more complete understanding. Generalizations made on patterns or
interactions between grazers and seaweeds are often based on few individual
studies and need to be more thoroughly tested with different species and at multiple
locations. This will allow us to better understand the variability of individual
patterns within a (possibly) general concept. Obviously, globally occurring changes
in climate and ocean chemistry are likely to have effects on the seaweed-grazer
systems as we have started to understand them so far. Aside from metabolic
changes in both grazers and seaweeds, marine systems are expected to undergo
“functional homogenization,” where numerous specialist species fulfilling various
functions are replaced with few generalist species assuming most ecosystem
functions (Clavel et al. 2011 ). This homogenization will inevitably have important
effects on the often species-specific nuances in seaweed-grazer interactions, evok-
ing important questions of the role of the diversity of these myriad interactions in
ecosystem resiliency and functioning.
Acknowledgements I am grateful to Brenda Konar for productive discussions and thoughtful
comments on the manuscript. I also thank Charles Amsler, Gunilla Toth, and Tania Spurkland for
helpful comments that greatly improved the manuscript. Preparation of the manuscript was
supported by NaGISA, the nearshore field project of the Census of Marine Life program, award
#2008-12-1 by the Alfred P. Sloan Foundation.
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