Physical Environment Salinity Usually expressed in ‰ (g per kg) “Average” sea water ~34.7 ‰

1. Physical Environment • Salinity • Usually expressed in ‰ (g per kg) • “Average” sea water ~34.7 ‰ • 33-37 ‰ in open ocean (very consistent) • Fresh water < 0.5 ‰ • Brackish water – 0.5-17 ‰ • Consistent in open ocean • Variable in shallow areas and near sources of fresh water • Ion composition consistent throughout ocean • Osmoregulation energetically expensive • Most marine organisms are osmoconformers

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5. Physical Environment • Pressure • Increases predictably with depth • 10 m = 1 atmosphere (14.7 psi) • Pressure @ 3700 m = 5450 psi • Affects gas-filled structures • Constrains vertical movements of many animals

6. Physical Environment • Density • Sea water 830x denser than air • Reduces need for substantial support structures • Marine organisms have adaptations for buoyancy, not support • Examples: • Fishes have light, flexible bones and swim bladders filled with gas/lipid • Whales have hollow bones filled with lipid • Most marine plant species are microscopic, floating organisms • Many marine macrophytes use gases for buoyancy • Many organisms live in permanent suspension (bacteria, viruses) • High density of sea water also influences: • Locomotion (less energy required than in air) • Feeding (enables suspension feeding) • Communication (efficient sound transmission)

7. Physical Environment • Gas Availability • Oxygen concentrations in sea water much lower than in air (~0.001% vs. 21%) • More soluble at lower temperatures • Concentrations affected by biological activity • Aerobic organisms may be limited in design, metabolic activity and distribution by • Oxygen availability • Ability to carry out rapid gas exchange

9. Physical Environment • Viscosity • Stickiness to objects due to adhesion • Objects moving through water drag some water with them • Many aquatic animals have adaptations to manipulate drag • Reduce: fast-swimming species (squid, tuna) • Use: some suspension feeders • Link

10. Physical Environment • Surface Tension • Stickiness at the surface due to cohesion • Water surface can support weight of very small organisms • Air-water interface supports numerous organisms • Microbes in surface films Halobates Fig. 4-2

11. Physical Environment • Water vs. Air • Dehydration not a problem (most terrestrial organisms expend energy avoiding desiccation) • Some marine organisms expend energy getting rid of water • Nutrient Availability • Food availability decreases with depth • Sea water contains many dissolved chemicals that can be absorbed directly

12. Zonation • Important concept in biological oceanography • Construct created by humans, but zonation has a basis in ecological reality • Pelagic/Benthic • Pelagic = “open sea” • Benthic = “bottom” • “Reverse benthic” • Neritic/Oceanic • Neritic zone overlies continental shelf • Bounded by land and 200 m isobath (typically), representing edge of continental shelf • Exception: Ross Sea (shelf to 800 m depth) • Some neritic zones are wide (E. Coast of US); others are narrow (W. Coast of S. America) • Oceanic zone overlies deeper water • Overall, 92% of World Ocean is oceanic

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14. Zonation • Depth Zones • Limits subjective & variable, ecologically-based • Pelagic • Epipelagic (0-200 m) • Definition based mainly on downwelling light intensity • Functionally similar to euphotic zone • Nearly all photosynthetic marine organisms • Epipelagic organisms adapted to well-lighted environment with few horizontal obstructions • Air-sea interface important to some organisms

15. Zonation • Depth Zones • Pelagic • Mesopelagic (200-1000 m) • Extends from lower limit of epipelagic zone to nominal depth of maximum sunlight penetration • Organisms typically non-photosynthetic • Some may migrate into surface waters periodically • Organisms adapted to low-light, fairly high-pressure environment with limited food availability and relatively cold temperatures • Bathypelagic (1000-4000 m) • Extends from lower limit of mesopelagic zone to ~mean depth of ocean • Organisms typically non-migratory • Low densities and difficulty of sampling limit knowledge of organisms

16. Zonation • Depth Zones • Pelagic • Abyssopelagic (4000-6000 m) • Conditions similar to bathypelagic zone • Organisms living in association w/sea floor show very different adaptations vs. purely pelagic organisms • Bottom-associated (benthopelagic, nektobenthic) animals often active scavengers and/or predators, skilled at detecting, localizing & utilizing scarce food • Benthopelagic species typically w/larger eyes and less watery tissues than species not associated with sea floor • Hadopelagic (6000-11,000 m) • Includes trenches; maximum depth of ocean • Pelagic trench fauna very poorly studied • Food resources often more plentiful than in abyssopelagic zone

17. Zonation • Depth Zones • Benthic • Supralittoral (Above mean high water) • “Splash zone” (Ex: La Bufadora) • May be immersed during storms and unusual high tides • Organisms adapted to almost constant exposure to terrestrial conditions but also must be able to cope with aquatic conditions periodically • Conditions may be highly variable • Harsh conditions  few organisms in this zone • Littoral/Intertidal (Mean high water to mean low water) • Immersed at high tide, exposed at low tide • Organisms must be able to cope with complete exposure and complete immersion every day • High light levels may support extensive populations of photosynthetic organisms