Biology Reference
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very abundant, and macroalgae were easily controlled by consumption rates
(Hauxwell et al.
1998
; Fox et al.
2009
,
2012
).
Another well-studied case of bottom-up and top-down controls of macroalgal
blooms is that of Venice Lagoon. In the 1970s and 1980s, the lagoon received
inputs of nutrients from urbanized areas in and around Venice from agricultural,
industrial, and treated and untreated sewage effluent sources (Sfriso et al.
1992
).
With high nutrient loading came noticeable changes in benthic community struc-
ture driven by large blooms of
Ulva rigida
and other green macroalgae (Sfriso et al.
1987
,
1992
). Between 1987 and 1998, macroalgal standing crop in Venice Lagoon
declined to only 1.6% of what was present in 1987 (Sfriso et al.
2003
). This
dramatic reduction was initially thought to be due to a combination of changes in
climate, sedimentation fluxes, and management of nutrient loading entering the
lagoon. Additionally, as macroalgal growth declined, fewer anoxic events allowed
for the recovery of invertebrate grazers, which were able to help control macroalgae
blooms from the top-down (Balducci et al.
2001
).
Other biotic and abiotic factors, however, may affect the relative roles of
bottom-up and top-down controls of macroalgal communities in estuarine systems.
For example, reproduction and recruitment of early life history stages of
macroalgae may respond differently under nutrient enrichment and grazing pres-
sure than adult life stages (Lotze et al.
1999
,
2000
,
2001
; Lotze and Worm
2000
).
Lotze et al. (
1999
) found that the bottom-up and top-down controls on early life
stages may act as a bottleneck for bloom-forming species of macroalgae in some
cases. Lotze et al. (
2001
), however, showed that total recruit density of ephemeral
bloom-forming macroalgae
Ulva
and
Pilayella
spp. in the Baltic Sea was positively
influenced by nutrient enrichment, while grazing only limited recruitment and
growth of the more palatable of the two species without changing the total recruit-
ment (Lotze et al.
2001
).
Macroalgal community structure may also influence the strength of bottom-up
and top-down controls. In the presence of canopy-forming macroalgal species, such
as
Fucus vesiculosus
, the response of ephemeral algae to nutrients was found to be
limited by as much as 90% compared to those without canopies due to a reduction
in light availability (Eriksson et al.
2007
). In contrast, the presence of epiphytes
growing on macroalgae may actually stimulate macroalgal growth if epiphytes are
preferentially consumed by grazers (Kamermans et al.
2002
; see also Chap.
11
by
Potin). Furthermore, on a smaller spatial scale, the presence of grazers within the
macroalgal canopy may also be an additional source of nutrients through their
excretion (Taylor and Rees
1998
, see also Chap.
4
by Gordillo).
Hydrodynamics can also alter the strength of bottom-up and top-down controls
on a system. For example, in Mondego Estuary, Portugal, mitigation measures to
improve the hydrodynamics of the estuary have been found to alleviate macroalgal
blooms occurrences caused by high nutrient loading by increasing the circulation
and diverting inflow of nutrient-rich waters (Lillebø et al.
2005
). In San Antonio
Bay, Argentina, high nutrient loads enter the bay exposing macroalgae to
elevated nutrient concentrations during low tide and supporting a large macroalgal
biomass (Teichberg et al.
2010
; Martinetto et al.
2010
,
2011
). Additionally, large
