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Day 10, August 10, 2005 TER daily cruise report
All this sunny weather has given us plenty of opportunities to measure light in the sea around the Tortugas.
This involves making light measurements at increasing depth (a profile) by lowering a light meter over the side of the ship. The amount of photosynthetically active light, measured in micro-Einsteins, or the flux of photons though a square meter plane (YIKES), is measured at one meter intervals to a set depth. This provides us an estimate of the energy available to water column phytoplankton, as well as seagrass and algae living on the seafloor. Light is reduced as water depth increases and this reduction is increased by the presence of suspended particles in the water column. In the clear water around the Tortugas, the photic zone, or the depth at which plants and algae can survive is quite deep. A consequence of this is that seagrasses, that require a considerable amount of light to survive, can grow on relatively deep portions of the shelf. Algae which require relatively little light can grow at great depths indeed. This allows these benthic (seafloor) plants to grow over much of the area of the shelf around the Tortugas providing food and dissolved oxygen to many of the animals that live on and in the bottom sediments. Benthic animals: worms, crabs, shrimp, snails, clams and starfish in turn provide food for predatory fishes, like cusk eels that live in burrows in the bottom, flounder that live on the bottom and various species of reef fish that rest on the reef during the day and at night move out onto the open shelf to feed.
Tonight we will conduct the first or our evening dives to document the gathering
of fishes at the edge of the reef and, as darkness falls, their migration
onto the shelf to feed. We will collect information on numbers of fishes,
what species undertake this migration, behaviors involved and for evidence
of cost in terms of mortality due to large pelagic predators.