Life in the Open Sea: Explore the Fascinating Ecosystem of the Open Ocean

1. Who, What, When, Where, Why, and How of The Open Ocean • Who inhabits the open ocean? • What defines the open ocean? • When do organisms live in the open ocean? • Where (which oceans) can you find open ocean? • Why is the open ocean a necessary ecosystem? • How does the open ocean interact with other marine ecosystems?

2. Chapter 17 The Open Sea

3. Regions of the Open Sea • The open ocean lies beyond the neritic zone • Vertical zonation depends on penetration of sufficient sunlight to support photosynthesis • photic zone: receives enough light for phytoplankton to survive • can extend to a depth of 200 meters (660 ft) in clear tropical waters • Epipelagic zone (corresponds to the photic zone): the location of pelagic animals in the upper 200 m of the ocean • Aphotic zone: light rapidly disappears until the environment is totally dark

4. Life in the Open Sea • Two groups of organisms inhabit the oceanic zone: plankton and nekton • based on productivity, biomass, abundance and diversity, plankton far outweighs nekton in open ocean • Classification of plankton • Plankton can be classified into logical groups based on: • taxonomy • motility • size • life history • spatial distribution

5. Life in the Open Sea • Classification of plankton • Classifying them into different groups: • phytoplankton: primary producers • zooplankton: heterotrophic eukaryotic microbes that float in the currents • bacterioplankton: archaeans and bacteria • viriplankton: free viruses (the most abundant plankton of all)

6. Life in the Open Sea • Classification of plankton • Classifying based on motility • akinetic: plankton that don’t move at all, e.g., viruses, diatoms • kinetic: plankton that can move (include majority of plankton) • kinetic plankton move by use of flagella, jet propulsion, undulation, swimming appendages

7. Life in the Open Sea • Classification of plankton • Classifying based on size • original scheme (based on visibilty and collection method): • macroplankton – visible to the naked eye • microplankton – caught with standard plankton net • nanoplankton – concentrated by centrifugation • newer classifications: • Femtoplankton – less than 0.2µm • Picoplankton – 0.2-2.0µm • Mesoplankton – 2.0µm-20mm • Macroplankton • megaplankton

9. Life in the Open Sea • Classification of plankton • Classifying based on life history • holoplankton: organisms that are planktonic throughout their lives, e.g., microbes, arrowworms, salps, siphonophores, comb jellies, copepods, krill • meroplankton: planktonic larvae that will grow into non-planktonic organisms • in open ocean would include larvae of nektonic fish and squid • in coastal waters would also include larvae of benthic invertebrates

11. Life in the Open Sea • Classification of plankton • Classifying based on spatial distribution • neritic: distinguished by presence of meroplankton and diverse diatoms • oceanic: less diverse in diatoms and invertebrate meroplankton; more salps, larvaceans, arrowworms and sea butterflies • neuston: plankton that live close to the water’s surface • pleuston: plankton which break the surface of the water with their gas bladders or bubbles, e.g., by-the-wind sailor

12. Life in the Open Sea • Patchiness in the open sea • plankton occur in patches (localized aggregations), often around upwellings • patchiness can be caused by: • upwelling • localized variations in sea surface conditions • vertical mixing • downwelling • waters of different densities coming together • grazing by zooplankton

13. Life in the Open Sea • Patchiness in the open sea (continued) • micropatchiness occurs throughout the photic zone when marine microbes become attached to particles of organic matter, esp. marine snow • marine snow: strands of mucus secreted by zooplankton that form translucent, cob-webby aggregates

14. Life in the Open Sea • Plankton migrations • many open-ocean zooplankton migrate daily from the surface to nearly 1.6 km deep • provides access to phytoplankton in the photic zone • reduces predation by plankton-eating fishes in the epipelagic zone • deep scattering layer: a mixed group of migratory zooplankton and fishes that are densely packed • can give sonar false image of a solid surface hanging in mid-water

15. Life in the Open Sea • Megaplankton • most organisms classified as megaplankton are animals • cnidarian zooplankton • largest members of the plankton are jellyfishes

18. Life in the Open Sea • Megaplankton • molluscan zooplankton • pteropods (sea butterflies) have a foot with 2 large wing-like projections and a greatly reduced or absent shell • purple sea snails produce bubble rafts • some species of nudibranchs

19. Life in the Open Sea • Megaplankton (continued) • urochordates • salps have barreled shaped bodies opened at both ends • composed of 95% water, hence grow and reproduce rapidly

20. Life in the Open Sea • Megaplankton (continued) • urochordates • larvaceans – only group of pelagic tunicates

21. Life in the Open Sea • Nekton • invertebrates • squids: reign supreme in open ocean as formidable predators • fish • billfish: species with an enlongated upper jaw (bill) and no teeth (e.g., marlin, sailfish, swordfish) • tuna: most wide-ranging of open ocean fishes, lack swim bladder – must swim constantly • ocean sunfish: feed on large zooplankton, especially jellyfish, have few natural predators • sharks: most efficient predators of open ocean • manta rays: have labial flaps which channel small fish and plankton into their mouths

24. Life in the Open Sea • Nekton (continued) • reptiles • Include sea snakes and sea turtles

25. Life in the Open Sea • Nekton (continued) • birds and mammals • penguins of Southern Ocean • whales • baleen whales filter krill, pteropods and fish • toothed whales feed on squid and fish

26. Survival in the Open Sea • How do organisms remain afloat? • swimming methods • flagella, cilia, and jet propulsion • dinoflagellates, coccolithophores, silicoflagellates, and blue-green bacteria swim with flagella • tintinnids, ciliates, and larvae use cilia • jellyfish, siphonophores, salps, and squid use jet propulsion • appendages • appendicular swimmers: organisms that use appendages to swim (e.g., copepods, pteropods) • undulations of the body • e.g., arrowworms, larvaceans, worms, fish, whales

27. Survival in the Open Sea • Remaining afloat (continued) • reduction of sinking rates • frictional drag: can be increased by decreasing volume, flattening the body or increasing body length • adaptations that increase friction do not prevent organisms from sinking, they merely slow the process • buoyancy: increased by storage of oils, increasing water content of the body, exchange of ions, and use of gas spaces

30. Survival in the Open Sea • Avoiding predation • due to lack of accessible refuges in open ocean, pelagic organisms have evolved a variety of adaptations to avoid predation • benefit of being less conspicuous • countershading: having dorsal surfaces that are dark blue, gray or green and ventral surfaces that are silvery or white • many planktonic species are nearly transparent

32. Survival in the Open Sea • Avoiding predation • safety in numbers • animals such as siphonophores (e.g., Portuguese man-of-war) increase chances of survival by forming colonies • looks like single individual • made up of thousands of individuals

33. Ecology of the Open Sea • Open sea is a pelagic ecosystem—one in which the inhabitants live in the water column • basis of food chain is many species of small phytoplankton • Small, primary producing organisms have a relatively high surface area • allows them to absorb more nutrients from surrounding seawater • Majority of herbivores in open ocean are zooplankton which supply food for nekton

34. Ecology of the Open Sea • Productivity • all higher forms of life rely on plankton • water near the surface receives plenty of sunlight, but few nutrients from land or the sea bottom (except in rare areas of upwelling) • phytoplankton productivity is low in tropical waters • arrangement of water in layers with little circulation between prevents nutrients from being brought from the sea bottom • low phytoplankton numbers support even fewer numbers of zooplankton

35. Ecology of the Open Sea • Food webs in the open sea • basis of food webs in open sea is formed by phytoplankton and heterotrophic bacteria • dissolved and particulate organic matter • phytoplankton release photosynthetic products as DOM into surrounding seawater • heterotrophic bacteria recycle DOM as they eat it and then are eaten by nanoflagellates • bacterial loop: process in which bacteria metabolize DOM and return it to the water in an inorganic form available to phytoplankton • lysis of bacteria by viruses releases DOM and particulate organic matter (POM)

39. Ecology of the Open Sea • Food webs in the open sea (continued) • efficiency of open-ocean food webs • conversion of biomass from one level to the next is surprisingly efficient • entire phytoplankton or bacterial production may be consumed daily by next trophic level • conversion rates (food to biomass) may be high • food webs may have food chains with 5-6 links • few large animals are supported away from upwelling areas because of limited rate of primary production and declining conversion efficiency along the food chain