lagoons has created a vast array of physicochemical and ecological gradients and microhabitats crucial in supporting fisheries and humans. Coastal lagoon complexes exist in many dry tropical regions, originating as wave‐cut terraces when sea‐levels were lower during the Pleistocene glaciations (Eisma 1997). In the Arabian Gulf, for instance, marine terraces or ‘sabkhas’ surround these high salinity lagoons. Aeolian dunes migrate across the terraces under the influence of NW or ‘shamal’ winds. Other high salinity lagoonal pools are equally ancient, formed by similar sea‐level changes isolating areas behind raised coral reefs receiving a subterranean supply of seawater seeping through coral stone.
Not all coastal lagoons are hypersaline. Large stretches of the Pacific coast of Mexico consist of lagoons frequently lying between rivers and connected by ‘esteros’, narrow and winding sea channels which permit ocean water to enter as a typical salt wedge and having all the characteristics of stratified estuaries. Salinities vary in relation to the dry and wet seasons. Lagoons in the wet tropics are frequently oligohaline for long periods of time. The lagoons along the north coast of the Gulf of Guinea (Ivory Coast) are situated in an equatorial climate where the annual rainfall is about 2000 mm. In Ebrié Lagoon, the largest of three main gulf systems, temperature varies little, but salinity varies with season and in different parts of the lagoon, ranging from euryhaline to oligohaline (Albaret and Laé 2003). The lagoon, like most lagoons worldwide, is frequently deoxygenated by pollution and by lack of circulation in the deeper areas. Coastal lagoons experience forcing from river inputs, wind stress, tides, the balance of precipitation to evaporation, different salinity regimes, and many human‐induced changes, all of which make each lagoon unique. This is probably why no universal classification scheme for coastal lagoons has ever been developed.
Abiotic factors are central to understanding the myriad properties of coastal lagoons. Flushing of a lagoon maintains water quality and physicochemical conditions and provides a mechanism for the import and export of nutrients, plankton, and fish. The overall characteristics of a lagoon are determined by salt and heat fluxes controlling warming and cooling. Geomorphological factors that play important roles in coastal lagoons include inlet and outlet configuration, lagoon size and orientation with respect to wind direction, bottom topography, and water depth. The size of the inlet/outlet controls the exchange of water and associated dissolved and suspended material and biota. The effects of sand bar openings can have a significant effect on physicochemical variables but can also have effects on the biota. For instance, the spatial variation in pH, dissolved oxygen, and nutrients in a hypertrophic coastal lagoon in Brazil (the Grussai lagoon) was linked to anoxic and nutrient‐rich groundwater discharge, the development of aquatic macrophytes, the biological activities of the phytoplankton community, and marine inputs (Suzuki et al. 1998). Whenever the sand bar closes, and the lagoon is cut off from the sea, the lagoon water becomes supersaturated with dissolved oxygen, exhibiting high pH and chlorophyll a, and low levels of dissolved nutrients. When the passage re‐opens, there is an enrichment of dissolved inorganic nutrients and a decrease in pH and in dissolved oxygen. Within a few days, marine conditions return suggesting that biological mechanisms in the lagoon are highly efficient. Groundwater can play an equally important role in forcing physiochemical conditions in some lagoons. For example, there are two different types of groundwater in the Celestun Lagoon, Mexico: one derived from springs within the lagoon and a second characterised by moderate salinities compared to the low‐salinity groundwater, mixed lagoon water, and seawater (Young et al. 2008). Groundwater discharge occurs through small and large springs scattered throughout the lagoon and the relative proportions of low versus moderate salinity groundwater vary over the tidal cycle. Substantial groundwater discharges can occur during both the dry and rainy seasons and can have a huge impact on nutrient concentrations and salinity in the lagoon.
The main boundaries of the coastal ocean (Figure 4.8) encompass the upper limit at the tidal freshwater zone (1) down to the river, estuary, and adjacent inner shelf waters, (2) with the seaward limit at the coastal boundary layer, (3) which is often delineated by a tidal front. These areas comprise the coastal zone where the seaward limit is dynamic, oscillating over time and space, especially in the wet season when it is displaced further seawards and the actual boundary layer breaks down. Boundary layers are formed when turbid coastal waters are mixed and trapped along the coast during calm conditions. These boundary layers break down not only during periods of high river discharge but also during periods of sustained strong winds. The coastal zone varies greatly in length and breadth depending on the strength and characteristics of local coastal circulation, river discharge, shelf width, climate, and latitude. On a semi‐arid or arid continental shelf, the coastal zone may not be located close to shore as such shelves are often macro‐tidal, with mixing of inshore and offshore waters extending to 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.
