Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 42

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.