Biology Reference
In-Depth Information
21.5 Bottom-Up Versus Top-Down Control in Marine Coastal
Ecosystems
There are an overwhelming number of studies focusing on bottom-up versus top-
down controls of primary producer biomass and community structure in marine
coastal waters (Hauxwell et al.
1998
; Lapointe
1999
; Balducci et al.
2001
; Lotze
et al.
2001
; Smith et al.
2010
; Martinetto et al.
2011
; Fox et al.
2012
). Marine
communities are, in general, naturally regulated by bottom-up and top-down forces;
however, the increase in anthropogenic activities in the coastal zone has influenced
the relative strength of bottom-up and top-down controls on these communities.
In many instances, increased nutrient inputs seem to be the main driver of
macroalgal blooms (Valiela et al.
1997
; Hauxwell et al.
1998
; Lapointe
1999
),
while in others changes in grazing pressure are thought to be responsible (Hughes
1994
; Smith et al.
2010
).
In order to determine the relative influence of these two factors, many experi-
mental manipulations of nutrients and grazing have been carried out in marine
habitats under a range of different in situ conditions. Recent metaanalyses by
Burkepile and Hay (
2006
) and Gruner et al. (
2008
) have compiled the results of
these experimental studies to see whether any general patterns can be deduced
regarding the relative influence of top-down and bottom-up controls on these
ecosystems. They found that in most cases nutrients and grazers independently
affected producer biomass, but that the relative role of top-down versus bottom-up
control is context dependent, that is, it may differ by latitude, by type of marine
habitat, by the functional groups of algae or type of grazers in the system, by the
productivity of the ecosystem, or by the sampling method of producer response
(Burkepile and Hay
2006
; Gruner et al.
2008
). They also found that in some cases
there were interactive or synergistic effects, in which nutrient enrichment in the
absence of herbivores showed the highest productivity and percent cover of
macroalgae (Gruner et al.
2008
; Burkepile and Hay
2006
).
21.5.1 Latitudinal Patterns
Most reports from temperate latitudes addressing the forces behind macroalgal
blooms appear to suggest that nutrients were involved (Baden et al.
1990
; Valiela
et al.
1992
; Sfriso et al.
1992
; Raffaelli et al.
1998
), with the exception of rocky
shores (Lubchenco
1978
; Lotze and Worm
2000
). Similar studies from tropical
latitudes place less emphasis on nutrients and suggest large impacts of grazing on
macroalgal blooms (Lewis
1986
; Hughes
1994
), but opinions differ (Lapointe
1997
; Littler et al.
2006
). The metaanalysis of Burkepile and Hay (
2006
) showed
that in tropical marine habitats, macroalgae responded independently to both
nutrient enrichment and herbivore removal by increasing biomass or cover. Nutri-
ent effects were, however, smaller than the grazing effects, and only important
