2. Coral Reefs
Marlin J. Atkinson and James L. Falter Hawaii Institute of Marine Biology University of Hawaii Kaneohe, Hawaii, USA
[email protected] [email protected]
In: Biogeochemistry of Marine Systems (2003) K. Black & G. Shimmield [eds.], CRC Press, Boca Raton, Florida, pp. 40-64.
2.1 Introduction The motivation for studying biogeochemical cycles of coral reefs has been to understand how these marine ecosystems maintain high carbon production, turnover and deposition of calcium carbonate in low-nutrient oceans (D'Elia and Wiebe, 1990). Over the past 80 years, there have been a variety of ideas advanced to explain high carbon production. One of the prevalent views is that close physical relationships between heterotrophs and autotrophs (i.e. symbiosis) creates an ecosystem where nutrients are either retained within the biota, or recycled within the community. A more recent view, however, is that coral reefs produce mostly low quality organic carbon and inorganic carbon, requiring relatively little input of nutrients. The nutrients that are removed from the water column are exported as particulate material and recycled over spatial scales of at least 100's meters. All biogeochemical pathways, or reactions, described for pelagic systems occur in coral reef ecosystems (see D'Elia and Wiebe, 1990). The biogeochemistry of coral reef ecosystems, however, is different from pelagic systems in spatial arrangements, rates, stoichiometry and mechanisms that govern the reactions. These basic differences between reefs and pelagic systems are only recently becoming apparent. A major obstacle in advancing coral reef biogeochemistry has been that biogeochemical rates are normalized in different ways, and these rates are dependent on the scale of measurement. Biogeochemical rates in the pelagic systems are normalized to biomass (g dry wt, Chl a, N or P content, ATP, etc.); measurements are extrapolated to larger scales by linearly scaling relationships with biomass per volume. This approach is widely practiced. In reef systems, however, it is problematic to measure biomass per area because biomass penetrates the carbonate substrate and is spatially very heterogeneous. Yet, rates are determined for biogeochemical processes and normalized to biomass making extrapolations to the whole system nearly impossible. Further, and more fundamentally, the quantity of biomass is not necessarily linearly related to biogeochemical rates. Many reactions are also normalized to surface area, but even this rather simple approach is unsatisfactory when applied. Different approaches and methods of normalization make it difficult to develop quantitative biogeochemical models. Many papers report organism-scale experiments, in which transfers of biogeochemical compounds are described but not quantified; those papers are not referenced here because it is impossible to assess the quantitative significance of the processes they address.
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In a previous review, D'Elia and Wiebe (1990) described biogeochemical pathways and reported the organisms involved in each reaction; there were few rates reported, and those that were reported were per unit biomass. In this review, we try to advance the understanding of the basic processes controlling kinetics and provide some estimates of rates per area; we do not want to repeat the information in that review. We briefly summarize the known biogeochemical reactions occurring in reefs, we will then try to place them in the physical context of the reef, with some estimates of the rates. We will also recommend, considering recent findings, a relatively new approach in making measurements of biogeochemical rates.
2.2 Coral reef morphology and zonation Coral reefs are living structures that biogenically produce calcium carbonate; they maintain themselves at sea-level against the destructive forces of waves. Morphologies of reefs are shaped by the underlying geological structures, and net growth, accretion and dissolution of carbonate structures (Grigg et al., 2002). It is necessary to define basic reef morphology (Stoddart, 1969) because biogeochemical reactions are related to different morphology and zones. The fore-reef is a transition region between the reef and the ocean, sloping from deeper shelf to the shallowest reef (Fig 1). This slope can vary in steepness depending on the underlying topography and the upward growth of the reef. The fore-reef is usually comprised of spurs and grooves. Waves break and water moves through the grooves, constantly eroding the structure. The spur or buttress actively accretes, growing upward and outward to maintain the forereef (Fig 1). The fore-reef rises to the reef crest which is the shallowest part of the reef. This area, classically, is comprised of coralline and turf algae which can be exposed at low tide. The reef flat is flat and shallow; but can vary in depth depending on whether the reef has been uplifted or sunk from island subsidence and from spatial variation in rates of reef accretion. Reef flats can be dominated by a variety of organisms, from pure stands of coralline algae, to small coral, crustose coral, pure macro-algae and even pure soft corals and zooanthids. The community structure largely depends on wave energy (Grigg, 1998). Reef flats have been a convenient site for community scale metabolic studies; most research on biogeochemistry has 3
algal pavement reef crest
grooves
coral - algae
sand - rubble
fore reef slope
Figure 1. Top: Aerial photograph of a section of the barrier reef of Chuuk Atoll, Caroline Islands (7˚27'N, 151˚51'E). Picture courtesy of the Coral Reef Research Foundation. Dark lines on the fore reef indicate the presence of grooves. Bottom: Generalized sketch of fore-reef and reef flat for Bikini Atoll, Marshall Islands (11˚30'N, 165˚25'E) adapted from Munk and Sargent (1954).
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been in this zone (Smith and Marsh, 1973). Back-reefs are comprised of sand deposits and coral knolls, often reticulated in wonderful patterns. The classic reef flat progrades to a lagoon, which usually has patch reefs of varying sizes. These general morphological features are shown in Fig. 1. Fringing reefs on high islands show similar features with a fore-reef, reef flat, a sand depositional area, but no lagoon nor patch reefs. The basic morphological features and zones discussed above are readily apparent in air photos and even satellite images (Fig. 1). Generally, sand is bright (40-60 % reflectance of incident light) making it easy to identify sand areas; algae and coral are darker, reflecting only 515 % of light (Hochberg et al., 2002). Highly three-dimensional structures, including those dominated by coral, are very dark and surrounded by sand; note the dark regions identified in the air photo. Major algal taxa and coral are spectrally distinct, making it possible to map different bottom types (Hochberg et al., 2002).
2.3 Basic biogeochemistry 2.3.1 Carbon In this section we introduce the basic biogeochemical cycles of carbon, nitrogen, phosphorus, silica and iodine (Fig 2). It is important to remember that the standing stocks of carbon, nitrogen and phosphorus in the water column are orders of magnitude lower than standing stock in the biota or in the sediments (Table 1). Table 1 is designed to show the mass per square meter for the water column a meter above the reef, the biota and the carbonate framework a meter below or into the reef. The meter scale is simply for ease in scaling and calculations. For example, one could increase or decrease the thickness of the water pool or the sediment pool to equal the mass in the biota. Or, if one wanted to estimate the flux (mmol m-2 d1
) of compounds through each pool, the pool size could be divided by a turnover time: i.e.,
turnover of water (104 per day for a current of 0.1 m s-1), turnover time of biota (10-2 per day for 100 day turnover) and accumulation of the sediment pool (by 15 × 10-6 per day for 5 mm per year accumulation). For example, phosphate flux in the water would be 5000 mmol m-2 d-1, phosphate flux in the biota would be 1 mmol m-2 d-1 and in the sediments, 0.02 mmol m-2 d-1.
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NO2DOP, DON, DOC -
I , NH4+ ,
Si(OH)4, NO3-
,
CO2, HCO3-
A
A
POMAllo
POMMixed
B
2
550CO2 + 30NH4+ + HPO42-+ xSi(OH)4
1
SUBOXIC
5
6
CaCO3
POMAuto + PQ 550O2 + HPO24, NH4, CO2, Si(OH)4
2-
HPO4, NH4+, CO2, Si(OH)4 2O2 + NH4+ 7
4NO3- + 5CH2O 2-
ANOXIC
Ca2+ + CO32-
5
POMMixed POMAll
D
3
NO3-
OXIC
B
N2
C
2CH2O + SO4 2CH2O
8 9
4
NO3- + H2O + 2H+
5HCO3- + N2 + H+ + 2H2O
2HCO3- + HS- + H+ CH4 + CO2
Transfer Pathway A) Uptake of dissolved nutrients B) Excretion / Respiration C) Particulate organic matter uptake D) Particulate organic matter export Figure 2. Schematic of fundamental biogeochemical reactions occurring in coral reef systems. POM stands for particulate organic matter. Different types of POM are denoted by the subscripts Allo (allochthonous), Mixed (autochthonous), All (autochthonous + allochthonous), and Auto (solely autotrophic).
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CaCO3
10
Ca2+ + CO32-
Reaction Pathway 1) Net primary production 2) Nitrate assimilation 3) N2 fixation 4) Nitrification 5) Organic matter remineralization 6) Calcification 7) Denitirification 8) Sulfate reduction 9) Methanogenesis 10) Calcium carbonate dissolution
One would conclude that phosphate flux through the water column is greatest and therefore is the largest potential source for most compounds.
Pool
P
N
C
0.16-0.90a
< 7a
10-250a
autotrophic
50b
1350b
22,400b
heterotrophic
50c
~2000i
~5,000d
1400h (10,000)e,f,g,h
34,000i
300,000h
