Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 41

Wide variations in tropical rainfall, hydrography, geomorphology, and tectonics lead to the formation of many sedimentary habitats peculiar to the tropics. Expansive sandy beaches, mud banks, green and blue anoxic mud regions, mixed terrigenous‐carbonate bedforms, hypersaline lagoons, stromatolites and, more generally, extensive intertidal sand‐ and mud flats, mangroves, coral reefs, and seagrass meadows are characteristic of shallow, tropical seas. These habitats are created and altered by processes peculiar to themselves and linked to climate and oceanographic factors and the rate of terrigenous sedimentation.

Extensive sandy beaches and flats, mud flats, mangrove forests, coral reefs, and seagrass meadows are among the most iconic of estuarine and marine habitats and are distributed widely throughout subtropical and tropical latitudes. Intertidal sand and mud flats develop in conditions more quiescent than sandy beaches, fostering deposition of fine‐grain sediment (Eisma 1997). The global distribution of sandy shorelines (Figure 4.3) shows that 31% of the ice‐free world shoreline is sandy, with Africa having the highest presence (66%) of sandy beaches (Luijendijk et al. 2018). The global distribution shows a distinct relation with latitude and hence to climate, while there is no relation with longitude. The relative occurrence of sandy shorelines increases in the subtropics and from 20 to 40° latitude with maxima near 30 °S and 25 °N. They are relatively less common (<20%) in the humid tropics where muddy substrates are most abundant because of high river discharge and precipitation. The global distribution of sandy shorelines agrees with the earlier determination of latitudinal variation of sediments on the inner continental shelf (Figure 4.1).

Tidal flats have a range of complex sedimentary structures, such as cross bedding, lenticular bedding, and mud/silt couplets that reflect depositional history. Mud flats can be sheltered or moderately exposed and are commonly found in tropical estuaries, tidal inlets, and river deltas. Tidal flats occur in macro‐tidal settings where local areas of deposition occur where sedimentologic processes are active and stratigraphic sequences are developing. There are several types of tidal flats in macro‐tidal regions: low tidal sand flats, mud/sand slopes encompassing the low to mid intertidal zones, mangrove‐fringed mud flats, and high intertidal and supratidal salt flats. Hypersaline tidal flats have recently been found to be important storage sites for salt, sediments, carbon, and nutrient elements (Brown et al. 2021). Unlike tidal flats in micro‐ and meso‐tidal settings, physical processes dominate biological processes such as bioturbation in salt flats.

While tidal flats occur throughout the marine geosphere, roughly 60% (about 75 000 km²) of sand, rock, and mud flats occur within the low latitudes. Of a global total area of 127 921 km² (Murray et al. 2019), 49.2% of the world’s tidal flats are in Indonesia, China, Australia, the United States, Canada, India, Brazil, and Myanmar; about 70% are found on three continents: Asia (44% of total), North America (15.5%), and South America (11%). Tidal flats are declining in global area; 16% of tidal flats were lost between 1984 and 2016 (Murray et al. 2019). In addition to direct losses from coastal development, increased subsidence and compaction of intertidal sediments, reductions in sediment supply, altered sediment deposition and erosion rates, vegetation loss, coastal eutrophication, and sea‐level rise are also likely drivers of intertidal flat loss. Tidal flats have responded by local migration, but not quickly enough to offset ongoing losses. For example, the highly dynamic tidal flats of the Meghna River estuary in Bangladesh have migrated extensively since 1984, but now occur within only 17% of their initial extent despite expanding in area by 21% due to the rate of sediment delivery exceeding the rates of subsidence and sea‐level rise; seaward migration of tidal flats has been slow, influenced by altered sediment deposition patterns due to coastal development and local expansion of mangroves (Murray et al. 2019).

FIGURE 4.3 Global distribution of sandy shorelines. The coloured dots along the world’s shores represent the local percentage of sandy shorelines (light brown is sand, dark brown is non‐sand). The subplot to the right presents the relative occurrence of sandy shores per degree latitude where the dashed line shows the latitudinal distribution. The lower subplot presents the occurrence of sandy shores per degree longitude. The curved grey lines in the main plot represent the boundaries of the ice‐free shorelines. The underlined percentages indicate the percentages of sandy shores averaged per continent.

Source: Luijendijk et al. (2018), figure 1, p. 4. Licensed under CC BY 4.0. © Springer Nature Switzerland AG.

The global distribution of mangroves (Figure 4.4) indicates a tropical dominance with major latitudinal limits relating best to major ocean currents and the 20 °C seawater isotherm in winter (Bunting et al. 2018) with most mangroves occurring in Southeast Asia and the Americas, including the Caribbean. Both mangroves and seagrasses grow best in quiescent environments where hydrology is favourable for their development (Chapter 2). Estimates of global mangrove area range from 83 495 km² (Hamilton and Casey 2016) to 135 870 km² (Worthington et al. 2020). A global typology of mangroves has found that as of 2016, 40.5% of mangrove ecosystems were deltaic, 27.5% were estuarine, and 21.0% were located on open coasts, with lagoonal mangroves occupying only 11% of global mangrove area (Worthington et al. 2020); mangroves in carbonate settings represent just 9.6% of global coverage.

In contrast, the known area of tropical seagrass meadows is poorly constrained by large areas remaining unmapped and inconsistent methodology being used (McKenzie et al. 2020). The global area of seagrasses most likely is 160 387 km² with a moderate to high confidence (Figure 4.5), but possibly 266 562 km² with lower confidence (McKenzie et al. 2020). Seagrass meadows in the tropical Atlantic (44 222 km² with moderate–high confidence) and in the tropical Indo‐Pacific (87 791 km² with moderate–high confidence) make up 82% of the global total and tropical seagrass meadows make up 85% of the global total if the low confidence estimates are included (McKenzie et al. 2020).

Like seagrasses and mangroves, the global distribution of coral reefs (Figure 4.6) reflects the influence of long‐term environmental conditions that are most suitable for establishment and growth. Species richness of corals is greatest in the Coral Triangle in Southeast Asia (Figure 4.6, dark red area). Corals are found in three principal areas: the Caribbean, the Red Sea, including the Indian Ocean islands such as the Seychelles, and in the Indo‐West Pacific. Corals are found in a broad band throughout the tropics, although there are areas out of this band where warm currents permit the existence of corals, such as on the west and east coasts of Australia and as far north as the southernmost islands of Japan. Coral reefs cover about 600 000 km² of the global ocean, comprising only about 0.17% of total ocean area and roughly 15% of the world’s sea floor <30 m (Spalding et al. 2001). This small area belies their importance to the structure and function of the tropical coastal ocean as they are important areas of fisheries, marine biodiversity, and carbonate production.

FIGURE 4.4 Global distribution of mangrove forests. The green bars outside the box indicate relative distribution of mangrove with latitude (right) and longitude (bottom).

Source: Bunting et al. (2018), figure 4, p. 10. Licensed under CC BY 4.0. © MDPI.

FIGURE 4.5 Global seagrass area relative to the maximum potential seagrass area within each of the global seagrass bioregions which are represented by scaled circles. Seagrass area in each bioregion: 1. Temperate North Atlantic = 3229 km²; 2. Tropical Atlantic = 44 222 km²; 3. Mediterranean = 14 167 km²; 4. Temperate North = 1866 km²; 5. Tropical Indo‐Pacific = 87 791 km²; 6. Temperate Southern = 9112 km².

Source: McKenzie et al. (2020), figure 4, p. 7. Licensed under CC BY 4.0. © IOP Publishing.

FIGURE 4.6 Global distribution of coral reefs. The coloured areas indicate species richness of hermatypic coral reefs in each region.

Source: www.coralsoftheworld.org (accessed 3 January 2021). © Japanese Coral Reef Society.

Six major physical factors limit the development of coral reefs: temperature, light, depth, salinity, sedimentation, and emersion (Montaggioni and Braitwaite 2009). Hermatypic corals are found in waters bounded by the 20 °C isotherm with the lower temperature set at 18 °C for reef formation. Optimum reef development occurs at mean temperatures of 23–25 °C, although some corals can tolerate higher temperatures. The absence of reefs from the west coasts of Central and South America and the west coast of Africa is due to the upwelling of cold water. Reefs are also depth and thus light‐limited and do not form in waters > 50–100 m. Most grow in 25 m or less and are restricted to continental margins or islands due to light limitation. Light compensation depth is the water depth where light intensity is 1–2% of incident light at the ocean surface. Corals do not grow below 50–60 m in the Pacific but grow as deep as 100 m in the Caribbean due to greater light penetration.

Another factor limiting the development of coral reefs is salinity as corals have a narrow tolerance for salt. Freshwater runoff can occur in proximity to reefs which they can tolerate for short periods of time, but generally corals thrive in areas where there is little if any decreases in salinity and increases in sedimentation that clogs feeding structures and reduces available light by turbidity or mixing (Montaggioni and Braitwaite 2009). Corals can also tolerate short periods of exposure to air, but generally their growth is limited to the tide mark of mean low water.

There are four types of coral reefs: fringing reefs, barrier reefs, patch reefs, and atolls (Sheppard et al. 2018). While they all differ in their geomorphology, they are all part of a series of forms that develop in the same basic manner. Corals will grow where conditions are suitable, especially in clear shallow waters and they can grow along tropical rocky coasts to about 45 m depth. Corals grow upward until limited by emersion into air and begin to spread outward. Fringing and barrier reefs are found along continental coasts and off islands while atolls are mostly found in the Indo‐Pacific area. Atolls are oceanic and circular in shape with a series of sandy cays enclosing a deep lagoon. They form when a submarine volcano develops a fringing reef and as it sinks over time the coral will grow upward. The top of the volcano then subsides to eventually form a deep lagoon in the centre of a group of coral reefs.

Barrier reefs can be located further offshore with a broad, wide lagoon compared to fringing reefs. Patch reefs are generally oval along the axis of the prevailing winds and may have a sandy cay on the leeward side. In some areas where there is enough shelter, patch reefs can develop into islands where they become low wooded islands and may even have mangroves and seagrasses in a patchy lagoon.

Reefs display a variety of zonation patterns depending on the water depth, wave action, and exposure (Sheppard et al. 2018) but the ‘classical’ zonation pattern is of a reef front or slope culminating in a wave break zone, followed by a reef crest then a reef flat which leads to a back reef or lagoon (Figure 4.7). The reef front or slope extends from the low tide mark to deep water and it is here that coral growth is most rapid; the slope is dominated by large corals such as Acropora and Monastrea within the upper 15–25 m. Wave action and light intensity are reduced below this depth; light is reduced to only about 25% of the surface so only small branching corals predominate. At about 25–40 m depth, corals become patchy as light becomes scarce and there is some accumulation of sediment. Gorgonian corals can dominate at this depth range. The wave‐break zone and reef crest bear the full brunt of the waves and there is often a pattern of groove and spurs which forms because of the constant wave action. The reef crest zone is exposed at low tide and varies in width from a few m to tens of m and is dominated by very hardy coral species that can withstand strong wave action. The reef flat can be tens of m in length and is one of the largest areas of the reef by area. It receives less wave action than the more forward zones but is still exposed at low tide and consists of a wide mixture of corals and turf algae and can often have quiescent pockets of sandy patches where a variety of invertebrates and fish exist. The reef flat deepens into the back lagoon where unconsolidated sediment prevails and where there can be ‘bommies’ or hummocks of massive coral skeletons on which grow a variety of organisms, including young corals. The back reef can be exposed at low tide and often has a dominant biota of calcified green algae, such as Halimeda, along with various species of seagrasses and hummocks of corals, such as Porites. It extends outward from the shore to the lagoon and reef flat and may be any area in size from a few tens to hundreds of m in length. The back reef is shallow and sheltered from wave action. Here, water circulation is less rapid, and sediment tends to accumulate, contributing to poor coral growth; benthic invertebrates are common.

FIGURE 4.7 Schematic of an idealised coral reef showing various reef zones from the reef front to the back reef. Zones are not to scale.

The geological development of coral reefs is controlled by temperature, nutrient availability, hydrology, and changes in sea‐level and ocean chemistry. Most research has focused on sea‐level changes in relation to ancient reef development and evolution (Montaggioni and Braitwaite 2009). Changes in sea‐level are related to the availability of habitats suitable for coral reef development and such changes, when large enough, have triggered mass extinctions (Chapter 5).

Biotic controls play a role in reef development (Montaggioni and Braitwaite 2009). The evolutionary history of coral reefs shows an increase in biological disturbance such that there was an increase during the Cretaceous and Cenozoic in predators specialised for corals, including bioeroders and herbivores. These specialised organisms influenced the community structure of coral reef ecosystems. Such organisms limit the distribution and abundance of sessile organisms, such as corals, which require a stable substrate and quiescent sedimentological conditions.

Coastal lagoons can be most simply defined as natural enclosed or semi‐enclosed water bodies parallel to the shoreline. Lagoons are sometimes confused with other coastal ecosystems, such as estuaries and coral reef lagoons. Thus, coastal lagoons can be most precisely defined as ‘shallow aquatic ecosystems that develop at the interface between coastal terrestrial and marine ecosystems and can be permanently open or intermittently closed off from the adjacent sea by depositional barriers’ (Esteves et al. 2008). The waters of coastal lagoons can span the range of salinities from fresh to hypersaline depending on the balance of hydrological drivers, including local precipitation, river inflow, evaporation, groundwater discharge, and seawater intrusion through or directly via the depositional barrier.

The geophysical characteristics that contribute to the formation and maintenance of a coastal lagoon are important and help in identifying different types of lagoons (Eisma 1997). The first characteristic is whether the coastal lagoon has a connection to the sea. Some lagoons are lentic non‐tidal, that is, without permanent connection to the sea or lentic micro‐tidal, permanently connected to the sea. The second characteristic of a coastal lagoon is its origins. Most lagoons have originated from the flooding of lowland coastal areas due to the global rise in sea‐level during the Late Quaternary marine transgression (Esteves et al. 2008). Lagoons originating in this way generally have large surface areas and are located parallel to the coastline which increases the probability for marine intrusions through or over the depositional barrier. Other lagoons have originated by the build‐up of sediments at the mouths of rivers due to the working of waves and tides to form a barrier. Such lagoons have a branched configuration and a high perimeter to area ratio and are formed by the flooding of river valleys. Due to their geomorphology, high levels of dissolved and particulate materials from land enter such coastal lagoons.

Perhaps no other coastal environments are as complex as coastal lagoons. The heterogeneity of geomorphologies observed among coastal 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).