Macroalgae in Tropical Marine Coastal Systems - Seaweed Biology - page 323

in 1994). However, instead of a successional sequence of algal growth towards the
proliferation of canopy-forming macroalgae, the cover of algal turf decreased after
1994, and returned to the predisturbance levels within a decade (Adjeroud et al .
2009 ). These results suggest that the availability of vacant space was not sufficient
to cause a persistent increase in algal cover, and that other factors, such as a
reduction in grazing pressure or an increase in nutrients, may be necessary for a
regime shift to macroalgal dominance (McManus and Polsenberg 2006 ; Mumby
2009 ; see also Chap. 21 by Teichberg et al.). A similar story emerged from the
protracted loss of coral cover in Jamaica following two hurricanes, three bleaching
events, a reduction of grazing pressure (overfishing and disease), and a potential
pulse of nutrients which all contributed to a dramatic increase of algal cover, from
~4% in 1977 to ~92% in 1993 (Hughes 1994 ). Experimental studies, like that of
Hixon and Brostoff ( 1996 ), demonstrate the importance of herbivores on the
successional trajectory in tropical marine ecosystems. In conclusion, the impacts
that macroalgae can have on tropical marine communities are complex and are
dependent upon the identity of the herbivores present as well as the developmental
stage of the seaweed community (see Coral reefs below).
16.3.2 Diversity
Tropical marine ecosystems are among the most diverse ecosystems on our planet.
While invertebrates have received a great deal of attention during biodiversity
surveys and studies in the past, macroalgae have received much less consideration.
Some early studies that have assessed macroalgal diversity patterns across latitudi-
nal gradients have arrived at conflicting conclusions, finding high species diversity
at low and midlatitudes as well as decreasing diversity towards lower latitudes (e.g.,
Pielou 1977 ; Santelices and Marquet 1998 ). A more recent literature review by
Kerswell ( 2006 ) revealed distinct gradients in species and genus diversity world-
wide. When all macroalgal genera were considered, tropical regions had lower
diversity than temperate regions. Only when a more reef associated order like the
Bryopsidales was examined, diversity peaked at low latitudes. Furthermore, Konar
et al. ( 2010 ) confirmed the common trend of higher taxa diversity at midlatitudes
compared to low latitudes in the northern hemisphere, particularly in the intertidal.
Until recently, the coral reef seaweed hotspots of the Caribbean Sea were
considered to be at Diamond Rock in Martinique and the Pelican Cays in Belize.
However, on a recent expedition to the Saba Bank, Littler et al . ( 2010a ) collected
between 150 and 200 seaweed species from 17 different dive sites, demonstrating
that there is still much to discover. Their checklist of 98 taxa including
43 Rhodophyta, 26 Chlorophyta, 26 Phaeophyceae (Heterokontophyta), and three
Cyanophyta contained several unknown species as well as some previously unre-
corded seaweed communities. These communities were located at depths of
25-30 m and were dominated by green, brown, or fleshy red macroalgae. In contrast