Biology Reference
In-Depth Information
when herbivores were absent. This was not the case for temperate marine habitats,
which tended to vary in the relative effects of bottom-up and top-down control
based on the nutrient status of the system. In high nutrient environments bottom-up
effects were stronger, while in low nutrient environments top-down effects were
more significant (Burkepile and Hay
2006
).
One of the main explanations for these latitudinal differences in the control of
macroalgal blooms is the types and sizes of grazers and their grazing rates. Other
possible explanations may be the extent of nutrient loading, the macroalgal taxa
involved in different systems, the length of the growing season, and the range in
light intensity in temperate versus tropical coastal waters.
21.5.2 Dominant Control in Different Habitats
In this section, we provide some key examples from the literature defining the
relative roles of bottom-up and top-down controls within different marine habitats,
including estuaries and coastal bays, coral reefs, and intertidal and subtidal rocky
shores. Through these examples, we show the variation in macroalgal responses to
their controls, and also try to include some of the studies, which have paved the way
or stimulated the debate in this field.
21.5.2.1 Estuaries and coastal bays
The relative roles of bottom-up and top-down controls in estuarine systems have
been thoroughly studied due to the high degree of disturbance found in these
ecosystems as a consequence of coastal urbanization and development. This over-
development leads to higher nutrient inputs from point and nonpoint sources of
wastewater and fertilizers into coastal waters, while wetlands and forests that
generally buffer and act as a filter along the coastline are being removed at
alarming rates. Most sites receiving high nutrient enrichment generally are found
to be bottom-up controlled, whereas undisturbed low nutrient environments are
often found to be top-down controlled (Hauxwell et al.
1998
; Burkepile and
Hay
2006
; Fox et al.
2012
).
This general model of the relative importance of bottom-up and top-down
controls on macroalgal blooms in estuaries is again exemplified by the Waquoit
Bay example. Waquoit Bay macroalgal blooms are stimulated by high nitrogen
loading to its receiving subestuaries (Valiela et al.
1997
; Hauxwell et al.
1998
; Fox
et al.
2008
). In the high nitrogen loaded sites of Waquoit Bay, macroalgal growth
rates and biomass were highest (Hauxwell et al.
1998
). Additionally, more frequent
hypoxic events lowered the abundance of small crustaceans, the dominant grazers,
and therefore, grazing rates were too low to compensate for the increase in algal
biomass (Hauxwell et al.
1998
; Fox et al.
2009
,
2012
). In contrast, in the low
nitrogen loaded site macroalgal growth rates and biomass were lower, grazers were
