Biology Reference
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are cycled tightly within tropical ecosystems to prevent losses and enable a high
level of productivity. The higher light levels and temperatures of the tropics make
continuous productivity possible despite lower nutrient concentrations.
The increased supply of nutrients to the marine environment, resulting from
human activities (agriculture, development, sewage, etc.), has been linked to
increased algal growth in environments where they were previously a limiting
factor (see below). However, the concentration of nutrients in seawater is not
always directly related to macroalgal growth rates. For example, under low ambient
nutrient conditions, high water motion can also stimulate increased growth rates of
macroalgae (McCook 1999 ). Tropical macroalgae are very efficient nutrient users
that are capable of taking advantage of nutrient pulses through surge uptake
mechanisms. Additionally, they can store and recycle nutrients in their tissues for
extended periods of time (Fong et al. 2003 ). In tropical macroalgae, small increases
in nutrient concentrations can trigger an uptake response comparable to that of
bloom-forming species from temperate estuaries responding to much higher nutri-
ent concentrations (Kennison 2008 ; Fong et al. 2001 , 2003 ). The concept that
nitrogen and phosphorus are limiting nutrients in the tropical environment remains
a source of controversy (Bell 1992 ; Lapointe 1997 ; Bell et al. 2007 ). Results of
nutrient-enrichment experiments using macroalgae taken from coral reef and
seagrass habitats indicate that nitrogen limitation is common (e.g., Lapointe et al.
1987 ; Littler et al. 1991 ; McGlathery et al. 1992 ; Delgado and Lapointe 1994 ;
Collado-Vides et al. 2007 ). Several investigations have reached the same conclu-
sion that phosphorus availability limits the productivity of fleshy macroalgae in
oligotrophic reef waters (Lapointe et al. 1987 , 1992 ; Littler et al. 1991 ). However,
alkaline phosphatase activity (APA) enables some macroalgae to fulfill their phos-
phorus requirements by facilitating the utilization of organic phosphorus. These
mechanisms have been well documented in macroalgae growing in the inshore
regions of the Great Barrier Reef (GBR) where nutrient supply can be extremely
variable and dependent on inshore human activities (Schaffelke 2001 ).
16.2.3 Light
Light is the most important physical factor affecting primary productivity in the
marine realm. At low latitudes, light levels and water transparency are high due to
limited suspended sediments, particulate and dissolved organic matter, and phyto-
plankton. Additionally, very little PAR is lost due to limited scattering and reflec-
tion of light on the sea surface. Although light penetrates deeper in the water
column in the tropics than in temperate regions, light attenuation with depth still
creates a gradient of light quantity and quality (see also Chap. 1 by Hanelt and
Figueroa). Light in this way plays a structuring role in the distribution of tropical
macroalgae. Rhodophytes, like crustose coralline algae, are common throughout all
reef zones, from the intertidal to 80-90 m deep, where the light intensity has been
estimated at ~0.2% of the surface radiation (Van den Hoek et al . 1978 ), and even as
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