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brown algae ( Hedophyllum sessile and Laminaria complanata ) in intertidal pools
and subtidally on the rocky coast of Washington, USA. Lubchenco ( 1978 ) showed
that when snails were removed from tide pools where Chondrus crispus dominated,
green ephemeral species settled quickly and outgrew C. crispus .
Only more recently has the role of bottom-up control, in terms of nutrient supply,
been explored in rocky shores (Menge 1992 ; Nielsen 2003 ; Bokn et al. 2003 ;
Korpinen et al. 2007a , b ). A review by Menge ( 1992 ) discusses some of the reasons
the role of bottom-up controls on rocky shores has been neglected for so long,
including the overemphasis of physical stress in these environments, logistical and
methodological constraints of experiments, and different training backgrounds of
marine benthic ecologists. Possible explanations of how nutrient enrichment
cascades up the food web and affects the community structure of rocky shores
were also discussed using a case study along the Oregon Coast. In this environment,
nutrient loading from upwelling was thought to increase primary production and lead
to higher abundances of filter-feeding prey that then supported higher abundances
and feeding rates of predators (Menge 1992 ). Further studies have also shown that
increased phytoplankton due to upwelling events and nearshore hydrography drives
this bottom-up regulation of the benthic community (Menge et al. 1997 ).
The relative effects of bottom-up and top-down control on the rocky shore
macrophyte community along the same rocky shoreline were later experimentally
tested in intertidal pools by Nielson ( 2003 ). Small increases in nutrient supply
strongly increased the total percent cover of macroalgae and the diversity of benthic
macrophytes under low herbivore abundance, but not under high herbivore abun-
dance. This response in the macrophyte community was primarily due to an increase
in the functional group of corticated algae. The results of this study suggest that open-
coast marine intertidal communities are more sensitive to fluctuations in nutrient
regimes than previously thought, and that nutrient effects are likely to be amplified
where consumers are overexploited or naturally low in abundance (Nielsen 2003 ).
In both the above cases, bottom-up control of intertidal communities was driven
by natural sources of nutrients from upwelling events rather than anthropogenic
sources. However, other scientists have linked changes in the macroalgal commu-
nity structure of rocky shores to coastal eutrophication, where macroalgal
assemblages nearer to urbanized areas consisted of higher abundances of the
opportunistic chlorophyte Ulva spp . than in control sites (D´az et al. 2002 ; Worm
and Lotze 2006 ). Overall the consensus is that top-down controls are dominant on
rocky shores but that bottom-up controls can interact with top-down controls to
change patterns in macrophyte community structure and food web interactions.
21.6 Conclusion
There may be no general solution to the bottom-up top-down debate in macroalgal
communities, but rather different responses based on species or functional groups,
physical and chemical environments, and types of grazers. The fact that so many
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