180
absorption (Bulthuis and Woelkerling 1983) (Losee and Wtezel 1983; Dalla Via
et al. 1988; Cebrián et al. 1999; Brush and Nixon 2002), nutrient uptake, gas
exchange, or a combination of these factors (Sand-Jensen 1977; van Montrfrans
et al. 1984; Sand-Jensen et al. 1985).
Some animals (e.g., sea urchins, turtles, dugongs, manatees, and migratory
waterfowl) will graze directly on seagrass leaves. However, most seagrass biomass
is probably consumed via detrital pathways involving microbial degradation of
drifting wrack and organic carbon buried in the sediment, although there are few
quantitative estimates of the magnitude of the detrital flux (Heck et al. 2008).
Remineralization of the buried organic carbon can fuel secondary chemosynthesis
that supports invertebrate-microbial chemosynthetic symbioses that consume sedimentary sulfide (Burdige 2006), a potential seagrass toxin (Borum et al. 2005;
Holmer and Nielsen 2007), and plays an important role in the economically important spiny lobster fishery of the Bahamas (Higgs et al. 2016).
8.3.3 Interactions with Humans
No exploration of seagrasses would be complete without a consideration of human
impacts, both direct and indirect, on seagrass systems (Fig 8.6). The industrial fixation of atmospheric nitrogen into chemical fertilizers has revolutionized global agricultural production, but much of the added nutrients are eventually delivered to
coastal environments. Seagrasses are capable of exploiting dissolved nutrients in
both the sediments and water column and are probably not nitrogen limited in most
cases (Zimmerman et al. 1987). Consequently, the nutrient loading serves to fuel
growth of phytoplankton and nuisance algae, including epiphytes, that compete
with seagrasses for light (Batiuk et al. 2000). Deforestation, tilling, and landscape
hardening associated with urbanization also promote the transport of mineral sediments to coastal waters, which contributes to the attenuation of light propagation
through the water to seagrass leaves. The resulting deterioration in water quality has
resulted in catastrophic seagrass declines throughout the world (Short and WyllieEcheverria 1996). Since the water quality issues described above are quantitatively
unique to local watersheds, their impacts are amenable to local management.
Consequently seagrass losses have been reversed following improvements in water
quality (Tomasko et al. 2005; Greening et al. 2011).
Anthropogenic perturbation of biodiversity, and particularly the decimation of
top predators, can have comparable and sometimes greater impacts on seagrass survival than eutrophication alone, by releasing epiphytic algae from mesograzer control (Eriksson et al. 2009; Duffy et al. 2015; Hughes et al. 2016). These experiments
(and others cited therein) show that biodiversity is a strong predictor of fundamental
ecosystem processes controlling seagrass biomass that may be as important as bottom- up processes (e.g., light, nutrients, CO 2 , and temperature) in regulating seagrass productivity.
R.C. Zimmerman
absorption (Bulthuis and Woelkerling 1983) (Losee and Wtezel 1983; Dalla Via
et al. 1988; Cebrián et al. 1999; Brush and Nixon 2002), nutrient uptake, gas
exchange, or a combination of these factors (Sand-Jensen 1977; van Montrfrans
et al. 1984; Sand-Jensen et al. 1985).
Some animals (e.g., sea urchins, turtles, dugongs, manatees, and migratory
waterfowl) will graze directly on seagrass leaves. However, most seagrass biomass
is probably consumed via detrital pathways involving microbial degradation of
drifting wrack and organic carbon buried in the sediment, although there are few
quantitative estimates of the magnitude of the detrital flux (Heck et al. 2008).
Remineralization of the buried organic carbon can fuel secondary chemosynthesis
that supports invertebrate-microbial chemosynthetic symbioses that consume sedimentary sulfide (Burdige 2006), a potential seagrass toxin (Borum et al. 2005;
Holmer and Nielsen 2007), and plays an important role in the economically important spiny lobster fishery of the Bahamas (Higgs et al. 2016).
8.3.3 Interactions with Humans
No exploration of seagrasses would be complete without a consideration of human
impacts, both direct and indirect, on seagrass systems (Fig 8.6). The industrial fixation of atmospheric nitrogen into chemical fertilizers has revolutionized global agricultural production, but much of the added nutrients are eventually delivered to
coastal environments. Seagrasses are capable of exploiting dissolved nutrients in
both the sediments and water column and are probably not nitrogen limited in most
cases (Zimmerman et al. 1987). Consequently, the nutrient loading serves to fuel
growth of phytoplankton and nuisance algae, including epiphytes, that compete
with seagrasses for light (Batiuk et al. 2000). Deforestation, tilling, and landscape
hardening associated with urbanization also promote the transport of mineral sediments to coastal waters, which contributes to the attenuation of light propagation
through the water to seagrass leaves. The resulting deterioration in water quality has
resulted in catastrophic seagrass declines throughout the world (Short and WyllieEcheverria 1996). Since the water quality issues described above are quantitatively
unique to local watersheds, their impacts are amenable to local management.
Consequently seagrass losses have been reversed following improvements in water
quality (Tomasko et al. 2005; Greening et al. 2011).
Anthropogenic perturbation of biodiversity, and particularly the decimation of
top predators, can have comparable and sometimes greater impacts on seagrass survival than eutrophication alone, by releasing epiphytic algae from mesograzer control (Eriksson et al. 2009; Duffy et al. 2015; Hughes et al. 2016). These experiments
(and others cited therein) show that biodiversity is a strong predictor of fundamental
ecosystem processes controlling seagrass biomass that may be as important as bottom- up processes (e.g., light, nutrients, CO 2 , and temperature) in regulating seagrass productivity.
R.C. Zimmerman