Tropical Marine Ecology. Daniel M. Alongi
Strong tidal fronts
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