Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 11

What makes the marine tropics unique compared to seas of higher latitude? Tables 1.1 and 1.2 summarise many of the characteristics that this book will cover; clearly, there are many environmental attributes that are either unique to or are more common in the tropics. Several habitats attain peak luxuriance in the tropics, namely, mangrove forests, seagrass meadows, and coral reefs. Both ‘wet’ (or ‘humid’) and ‘dry’ tropical regions occur as do areas that undergo distinct ‘wet’ and ‘dry’ seasons. More research has tended to focus on what at first glance appears to be richer, wetter ecosystems, but areas and periods of aridity are more common than are reflected in the literature.

Spatial and temporal variations in rainfall and temperature are large in the tropics; daily thermal and precipitation changes increase away from the equator. The western boundaries of the tropical oceans are warmer, wetter, and more stable climatically than the eastern boundaries, caused by the asymmetrical form and unequal size of the ocean margins, which in turn strongly affect sea surface temperatures, currents, and nutrient regimes (Webster 2020) These geographic differences are of considerable ecological importance, influencing the distribution and abundance of shallow water habitats.

TABLE 1.1 Major hydrological and climatological characteristics unique to or dominant in the tropical oceans. Summary from Chapters 2 and 3.

Hydrology	Climatology
37% of world ocean area	High and stable solar radiation
69.1% of freshwater discharge to the world ocean	Absorbed solar radiation exceeds long‐wave radiation so net radiation balance is positive
Lower mean tidal amplitudes	High and stable temperatures
Small Coriolis parameter in proximity to the equator	Lowest and highest rates of evaporation and precipitation
Large Rossby radius	Trade winds (easterlies and westerlies)
Weak rotational constraint on bottom boundary layer	Absent/uncommon frontal storms within 5° of equator
Large buoyancy flux	Interannual variation > seasonal variation
Wind‐produced homogenous layer deepest in equatorial waters	Monsoons (dry–wet or arid): Asian, African, Indo‐Australian, and South American systems
DCRITICAL DEPTH > DWATER DEPTH	Tropical ocean absorbs most incoming solar energy
Seasonal upwelling	Tropical ocean‐atmospheric system is the heat engine of the global climate system
Permanently stratified thermoclines and haloclines; oxygen minimum layers	Hadley Circulation distributes equatorial winds in the low latitudes
Salinity and pH highly variable; acidic and hypersaline conditions common	Intertropical Convergence Zone, a belt of convective cloud about the equator. Zone of rising air and intense precipitation (accounts for 32% of global precipitation)
Estuarization of shelves by river plumes	Indo‐Pacific Warm Pool, an oceanographic/climatological phenomenon in the western Pacific Ocean; heat engine of the planet
Strong tidal fronts	Formation of tropical cyclones (typhoons, hurricanes)
Lutoclines (a front between two layers of comparatively high and low suspended sediment concentration) and high‐salinity plugs in estuaries and nearshore waters in dry season/arid regions	El Niño‐Southern Oscillation, large‐scale, global, coupled atmosphere–ocean system resulting in major surface climate anomalies throughout tropics
Tidal mixing, trapping, and complex small‐scale circulation in mangrove tidal waters	Indian Ocean Dipole, coupled ocean–atmosphere differences in convection, winds, sea surface temperatures, and thermocline causing large‐scale differences in rainfall patterns
Highly complex, small‐scale circulation on coral reefs and in hypersaline lagoons	Madden‐Julian Oscillation, a phenomenon that is a major source of intra‐annual variability in the tropical atmosphere, affecting monsoonal and cyclonic patterns
Indonesian Throughflow, unique feature passing warm and fresh Pacific waters into the Indian Ocean via the Indonesian Archipelago	Pacific Decadal Oscillation, dominant year‐round pattern of North Pacific sea surface temperature variability. Complex aggregate of different atmospheric and oceanographic forcings spanning the extratropical and tropical Pacific

The tropics form a band around the equator that comprises nearly 40% of the world’s open ocean area and over one‐third of its continental shelves (Table 1.1). As aforementioned, mangrove forests, coral reefs, and seagrass meadows constitute the richest habitats, but the drier tropical regions have hypersaline lagoons, stromatolites, and carbonate‐dominated shelf margins, the exemplar of the latter being the Great Barrier Reef shelf. Hydrological and climatological characteristics of marine tropical seas are in toto unique, reflecting proximity to the equator. Such physical characteristics include high and stable solar radiation and temperature, highest and lowest rates of rainfall, easterly trade winds, a large Rossby radius with low mean tidal amplitudes (one notable exception is the NW coast of Australia where mean tidal ranges can exceed 10 m), a small Coriolis effect resulting in a lack of cyclones, hurricanes, and typhoons close to the equator, permanently stratified shelf waters with strong tidal fronts, but with estuaries having high salinity plugs and lutoclines during the dry periods; interannual variation is greater than seasonal variability despite some regions having distinct ‘wet’ and ‘dry’ intervals.

At least 69% of all freshwater and 60% of all sediment discharged to the world’s coastal ocean do so via tropical rivers (Milliman and Farnsworth 2011; Laruelle et al. 2013). This phenomenon occurs primarily in the wet tropics and plays an important role as a driver of geological characteristics, oceanographic processes, and the structure and function of pelagic and benthic food webs. For instance, coastal waters receiving river water have a large buoyancy flux, and there are several regions where upwelled waters mix in a complex manner with discharged river water and associated materials, e.g. the southeast (the Gulf of Papua) and SW (the Aru Sea) coasts of New Guinea (Aller et al. 2004, 2008b; Alongi et al. 2012) producing an ‘estuarisation’ of the shelf margin with oxygen minimum layers and strong tidal fronts.

Geologically, intensely weathered silt and clay particles form muddy facies that dominate many inner and middle shelf margins, especially close to river deltas (Table 1.2); such shelves are ordinarily wide and shallow, and inshore areas have massive mud banks (‘chakara’) that migrate seasonally and annually as well as varying over decadal time scales (Gratiot and Anthony 2016). In drier regions where river discharge is small and/or highly seasonal, erosion is also highly seasonal but there is a high level of resolution of the geological record in which sedimentary facies are either carbonate‐dominated or mixed carbonate–terrigenous deposits, or both. Coral material and debris from other calcium carbonate‐bearing benthic organisms are most abundant in these areas; the extreme of this phenomenon is cementing dunes (‘sabhka’) in hypersaline lagoons. Throughout the ‘wet’ and ‘dry’ tropics, there are thus extremes of sediment accumulation and of burial of carbon and other elements. In many tropical regions, high rates of sediment erosion due to no or poor land‐use practices have resulted in rivers that are highly eroded with beds that are wide, shallow, and sand‐ and/or gravel‐dominated.

TABLE 1.2 Major geological and chemical characteristics unique to or dominant in the tropical oceans. Summary from Chapter 4.

Geology	Chemistry
60% of world’s sediment discharge from tropical rivers	Lowest organic carbon and nitrogen content in carbonate deposits
Mud and coral most abundant on inner shelves	Highest organic carbon and nitrogen content in mangrove muds, mud banks, and off river plumes
Intense physicochemical weathering of bedrock and soils	Low (μM) concentrations of dissolved inorganic nutrients
Many shelves wide, shallow (<120 m depth), and carbonate‐dominated open shelves or protected (‘rimmed’) lagoons	NO2– + NO3– and SO4– present in interstitial waters
Mixed carbonate–terrigenous sedimentary facies on shelf margins	Low O2 conditions (<5 mg/l) in estuaries, lagoons, and inshore waters
Migrating fluid mud banks (‘chakara’)	Benthic nutrient regeneration rates low
Cementing dunes (‘sabhka’) in hypersaline lagoons	Low interstitial water content, especially in carbonates
Highly seasonal erosion/deposition cycles	Dominance of iron and manganese reduction in sediment suboxic diagenesis
River beds highly eroded, wide, shallow, and gravel‐dominated	Particle coastings enriched in Fe‐, Mn‐, and Al‐oxides
Bioturbation mostly at sediment surface	Kaolinite and gibbsite common clay minerals
High resolution of geological record	Intense scavenging of dissolved oceanic components
Extremes in sediment accumulation and carbon burial rate	Photochemical processes important

Intense chemical and physical weathering of tropical soils results in their transfer to the marine environment, with the result being sediment particles rich in iron, manganese, and aluminium oxide coatings. The most common minerals in such highly weathered environments are kaolinite and gibbsite, and in the water column, there is intense scavenging of dissolved oceanic components as well as important photochemical processes. When fuelled by highly weathered but low concentrations of carbon and nitrogen (lignin‐rich) debris, this combination favours microbial decomposition pathways in sediments that are dominated by metal reduction. The latter is also fostered by high‐disturbance events especially near large tropical rivers where the seabed is shallow, and the benthos is dominated by near‐surface bioturbation and by small, opportunistic benthic infauna noticeably lacking in large, deep‐dwelling, equilibrium species of annelids and molluscs (Aller et al. 2008a), but with an abundant epifauna.

Distal to rivers, phytoplankton production and respiration can be just as high as in higher latitudes, but such production (mainly by small‐sized picoplankton rather than by larger diatoms and chlorophytes) is often displaced offshore due to high turbidity within plumes (Smith and DeMaster 1996; McKinnon et al. 2007). These rapid rates of productivity occur despite low (μM) concentrations of dissolved nutrients, comparatively low (≤ 5 mg/l) oxygen concentrations, and low rates of benthic nutrient regeneration. Pelagic food chains are arguably dominated by abundant macrozooplankton, mostly crustaceans such as penaeid shrimp, whose abundance and productivity yield a high percentage of crustaceans to finfish catch off tropical fishing grounds. Why crustaceans are so predominant in the low latitudes may lie in their genetics, competitive abilities with finfish or with life histories being simpatico with tropical oceanographic or climatological peculiarities, the latter of which we will explore in Chapter 2.