Q1) Compare the population densities of the Amazon and the Ganga Brahmaputra Basin.
Explain two reasons for the difference.
(2)
Answers
Answer:
Large rivers create major gaps in reef distribution along tropical shelves. The Amazon River represents 20% of the global riverine discharge to the ocean, generating up to a 1.3 × 106–km2 plume, and extensive muddy bottoms in the equatorial margin of South America. As a result, a wide area of the tropical North Atlantic is heavily affected in terms of salinity, pH, light penetration, and sedimentation. Such unfavorable conditions were thought to imprint a major gap in Western Atlantic reefs. We present an extensive carbonate system off the Amazon mouth, underneath the river plume. Significant carbonate sedimentation occurred during lowstand sea level, and still occurs in the outer shelf, resulting in complex hard-bottom topography. A permanent near-bottom wedge of ocean water, together with the seasonal nature of the plume’s eastward retroflection, conditions the existence of this extensive (~9500 km2) hard-bottom mosaic. The Amazon reefs transition from accretive to erosional structures and encompass extensive rhodolith beds. Carbonate structures function as a connectivity corridor for wide depth–ranging reef-associated species, being heavily colonized by large sponges and other structure-forming filter feeders that dwell under low light and high levels of particulates. The oxycline between the plume and subplume is associated with chemoautotrophic and anaerobic microbial metabolisms. The system described here provides several insights about the responses of tropical reefs to suboptimal and marginal reef-building conditions, which are accelerating worldwide due to global changes.
Explanation:
Because of their impact on salinity, pH, light penetration, sedimentation, and nutrients, large tropical rivers typically exclude carbonate reef builders from continental shelves. The Amazon-Orinoco and the Ganges-Brahmaputra mouths are textbook examples of such major reef gaps (2). The wide (~300 km) Amazon continental shelf evolved from a carbonate to a siliciclastic system during the early Late Miocene (9.5 to 8.3 million years ago) (5, 6). By this time, under lowstand sea level, an incised canyon system directed sediment influx toward the slope and basin floor (7). Shelf edge reef buildups occurred peripherally to this deep Amazon Fan and were gradually overlain by siliciclasts during Neogene and Quaternary highstands (7, 8). At present, the high sediment load from the river settles relatively quickly in the inner and mid shelves, conditioning an unstable muddy benthic habitat with high bacterial biomass and low diversity and abundance of epifauna and meiofauna (9, 10).