Tropical Marine Ecology. Daniel M. Alongi

mid‐shelf. In the wet tropics, the coastal zone often extends beyond the shelf edge, especially in proximity to large rivers, such as the Amazon.

FIGURE 4.8 Idealised scheme defining the coastal ocean and the coastal zone with some key biogeochemical fluxes linking land and sea and pelagic and benthic processes. The latter are not to scale.

      Source: Alongi (1998), figure 6.15, p. 184. © Taylor & Francis Group LLC.

Coastal circulation is driven by energy derived from solar heating or gravity, barometric pressure, and the density of oceanic waters (Section 3.3). Mixing results from tides, wind‐driven waves and buoyancy effects from river runoff, and mixing and circulation are thus greatly affected by geomorphology and bathymetry of the coastal zone. There are three main types of estuarine and coastal circulation: gravitational (due to river runoff), tidal (tidal pumping), and wind‐driven (Walsh 1988). Tidal circulation is usually the most important, with interaction by coastal boundaries generating turbulence, advective mixing, and longitudinal mixing and trapping, with the latter setting up coastal boundary layers. Coastal systems may be classified as tide‐dominated, wave‐dominated, or river‐dominated or a mixture of each, depending upon coastal geomorphology and local hydrography.

      The boundaries of the coastal ocean are somewhat arbitrary, driven by the energetics of a very dynamic sea. The coastal zone can extend to the shelf edge under extreme circumstances, but for the most part extends to the inner shelf. Oceanic and estuarine waters intermingle on the shelf proper and tongues of oceanic water regularly or irregularly intrude onto the outer shelf but can sometimes intrude as far as the middle of the continental shelf (Walsh 1988).

4.3 Nutrients

      Very sharp gradients in temperature, salinity, dissolved oxygen, and nutrients exist in tropical waters, partially reflecting high local and regional variability in precipitation and high solar insolation. Sharp thermoclines and haloclines coincide with strong vertical discontinuity maintained throughout most of the year, except where equatorial and coastal upwelling force cooler and more nutrient‐rich water to the surface, or where waters from central oceanic gyres intrude into humid regions to become warmer and more dilute. Vertical stratification often breaks down in shallow coastal waters, especially during the wet season, and during the dry season when trade winds are sustained. Great variability in salinity and its ability to adjust rapidly to changes in wind‐induced motion and temperature characterises tropical surface waters that are always warm (25–28 °C) and often less saline (33–34).

      The global distribution of sedimentary organic carbon and nitrogen is not related to latitude but dependent on water depth, grain size, terrestrial runoff, and hydrography (Alongi 1990; Burdige 2006). The highest concentrations of organic matter in sediments, as in the water column, are in regions of coastal upwelling and in proximity to rivers, and more generally contributes to patterns of pelagic primary productivity. While there are no latitudinal trends, highest sediment carbon and nitrogen have been measured in mangroves and seagrass meadows and the lowest in carbonate, mainly reef, deposits. In nearshore subtidal sediments, the highest values have been measured off the east and west coasts of India where mud banks occur and where organic pollution prevails. Carbon concentrations > 5% and nitrogen levels > 1% by sediment DW are not uncommon in tropical inshore muds and in vegetated deposits. Total phosphorous concentrations are also frequently high in areas of domestic waste, such as in many of the polluted estuaries of Southeast Asia.

Seasonal variations in particulate organic matter, particularly in estuaries, are greatly influenced by monsoonal rains. In Indian estuaries (Gireeshkumar et al. 2013) total organic matter and organic carbon (C) and nitrogen (N) levels decline during the monsoon season due to scouring of sediments during flood discharge. Low levels of organic matter and nutrients are common in carbonate deposits and are generally lower than in quartz sand and mud of equivalent grain size. The ratios of C:N and N:P (phosphorus) vary greatly in tropical sediments, as they do in other latitudes; these variations reflect the relative importance of terrestrial compared to the marine origin of the deposited organic matter (Gireeshkumar et al. 2013).

      Concentrations of dissolved inorganic nutrients are normally lower in tropical sediments than in temperate sediments of equivalent grain size. In tropical sediments, concentrations of all constituents are in the micromolar range, whereas interstitial nutrients are usually in the millimolar range in temperate sediments. Lower nutrient concentrations in sediments as well as in the water column reflect the fact that microbial decomposition and thus turnover of the nutrient pools are faster in the tropics due to warmer temperatures and highly productive microbial assemblages (Alongi 1990; Pratihary et al. 2009). It may also be partly due to phytoplankton communities that are of generally smaller size than the net‐sized phytoplankton of temperate waters, with generally less deposition of phytoplankton‐derived detritus to the seabed (Alongi 1990; Pratihary et al. 2009). This is reflected also in the fact that nutrient regeneration in the seabed is low compared with regeneration from temperate deposits. As in terrestrial ecosystems in the tropics, it is likely that nutrients in tropical marine ecosystems are tied up in living plant and microbial biomass.

4.4 Tropical River Loads, Plumes, and Shelf Margins

Tropical rivers of various sizes occupy a significant fraction of the world’s coastlines, but tidal rivers located in the wet tropics have the most impact on the geology and hydrology of the global ocean. 69.1% of the world’s river water (26 084 × 10³ km a⁻¹; Laruelle et al. 2013), laden with nearly 60% of the world sediment discharge (10 × 10⁹ tonnes a⁻¹), enters the tropical coastal ocean annually, mostly from the largest rivers (Table 4.1). The Amazon is the world’s largest river, but most tropical river water and sediment enters the global ocean from the Indo‐Pacific archipelago where high relief and rainfall produce high freshwater and sediment yields. Other major river/ocean boundary regions are in north‐eastern South America and west‐central Africa. The Amazon alone accounts for a disproportionate amount of the global flux, but the smaller mountainous rivers in Southeast Asia, ignored until recently, account for a greater proportion (43%) of the present sediment discharge estimates.

      Tropical estuarine (349.4 × 10³ km²) and watershed (58 707 × 10³ km²) areas constitute 34.5% and 52.0% of the world’s totals, respectively (Laruelle et al. 2013). Tropical continental shelf area (11 094 × 10³ km²) and volume (720 576 km³) constitute 36.6 and 18.7% of the world’s totals. The small percentage of shelf volume is due to the fact the tropical shelves are on average narrower and shallower than shelves of higher latitude (Laruelle et al. 2013).

      The dissolved loads of wet tropical rivers constitute about 65% of the world’s total (Huang et al. 2012). The proportion of water and sediment discharged from tropical rivers are a likely underestimate as many small‐ and medium‐sized tropical rivers remain ungauged (Latrubesse et al. 2005).

The relative importance of small mountainous rivers to the coastal ocean is exemplified on the islands of New Guinea and Timor. On the island of New Guinea, the ten largest rivers contribute only 35% to the island’s total river yield of 1.7 × 10⁹ t of sediment to the adjacent coastal zone as discharge from roughly 240 smaller rivers make up the balance (Milliman 1995). There are no large rivers on the much smaller island of Timor and the island discharges much smaller amounts of water (170 km³ a⁻¹) and sediment (133 × 10⁶ t a⁻¹), but area‐specific rates, including carbon and dissolved and particulate nutrients, are much higher than in New Guinea due to very high rates of deforestation and land degradation (Alongi et al. 2013). Borneo