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 is a special case where despite moderate relief the sediment yield is high (even under rainforest), probably because of continuous uplift.
TABLE 4.1 Estimates of water (km3 a−1) and suspended sediment (109 tonnes a−1) discharge from gauged tropical rivers. Rivers are ranked by water discharge.
Source: Alongi (1990), Milliman and Farnsworth (2011) and Liu et al. (2020). © John Wiley & Sons.
| River | Country | Water discharge | Sediment yield |
|---|---|---|---|
| Amazon | Brazil | 6300 | 1200 |
| Zaire | Zaire | 1300 | 43 |
| Orinoco | Venezuela | 1100 | 210 |
| Brahmaputra | Bangladesh | 630 | 540 |
| Mekong | Vietnam | 550 | 110 |
| Ganges | Bangladesh | 490 | 520 |
| Ayeyarwady | Burma | 430 | 360 |
| Tocantins | Brazil | 370 | 75 |
| Pearl | China | 300 | 69 |
| Zambesi | Mozambique | 220 | 20 |
| Salween | Burma | 210 | 180 |
| Fly | New Guinea | 170 | 120 |
| Niger | Nigeria | 160 | 40 |
| Magdalena | Colombia | 140 | 140 |
