A benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake that includes the sediment surface and its sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom and many such organisms are permanently attached to the bottom. The superficial layer of the soil lining the given body of water is an integral part of the benthic zone, as it influences greatly the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rock outcrops, coral, and bay mud.
The Hundred Islands National park is rich with wide variety of corals that provides shelter and food for invertebrates, fishes and other marine animals and plants (Map of Coral Reefs). Protected and conserved, these serves as foraging area for various marine animals. It can also be a good source of food, raw materials and recreation for humans. In recent years, both natural and man-made activities posed threat to this fragile marine environment.
A survey conducted by the PAWD and ERDS, DENR Region I in CY 2008 (Percent Cover of Major Benthic Life forms in the HINP) in eleven selected sites indicates a category of GOOD percentage reading of major benthic lifeforms in the Hundred Islands and in the City waters with 62 % with a live coral cover still intact. There are 11 dominant taxomony of hard and soft corals found across HINP these are: the Porites , Acropora ,Fungia , Seriatopora, Favia, Poscillopora , Pavona, Montipora, Millepora, Merulina and Heliopora. Live coral ranges from 22.9% to 84% with an average of 61.565% cover which can fall within the GOOD CATEGORY based from Gomez and Alcala’s (1979) classification.
Coral diversity was high with 41 genera of corals recorded in the 11 stations. These were observed as monospecific stands in some stations and as mixtures of branching, encrusting, massive, foliose and tabulate coral growth in other stations. The corals of the national park are found growing as reef patches with depth 5-10 feet and at the reef ledges and slopes at 10-45 feet depth.
The same report also revealed that there are about 82 fish species belonging to 30 families (Dominant Fish Species in the Hundred Islands National Park)
However, Identified natural threats observed in the City waters and within the Hundred Island National Park are coral bleaching from high water temperatures, (from both the short-term effects of El Nino and the long-term effects of global warming), diseases, grazing and predation by highly invasive plant and animal species, and tropical storms.
Likewise, human intrusive activities also poses a greater threat. Most identified practices are illegal fishing, overfishing, collateral damage from fishing and collecting marine organisms, erosion from land and resulting siltation, overstocking or overfeeding in mariculture resulting to eutrophication and fishkills. The unaddressed pollution brought by mismanaged solid , liquid and toxic waste resulting to fouling or overgrowth of algae and other invasive organism is also a growing concern. Global warming which will result to ocean acidification will also led to slower or no calcification and eventually destruction of coral reefs.