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The Marine Environment

Jon Thomassen Hestetun*, Kjersti Sjøtun*, Dag L. Aksnes, Lars Asplin, Jennifer Devine, Tone Falkenhaug, Henrik Glenner, Knut Helge Jensen and Anne Gro Vea Salvanes

The marine environment covers over 70% of the surface of the Earth, yet represents special challenges when it comes to scientific inquiry. When compared to terrestrial systems, the marine environment is much less easily accessible and, despite great effort, remains less well known. With the rise of the modern natural sciences, tools and methods have been continually developed to explore marine environments, from the littoral zone and nearshore environment to open waters and the shelf and abyssal seafloor. From tried and true collection equipment, often identical to or based on fishing gear, to new innovations in remotely controlled and autonomous vehicles, exploration of the underwater world is heavily dependent on the tools used.

Technological advancement now allows marine field studies to be conducted at all levels: from individuals to populations, to groups of populations, and to entire ecosystems. Habitats from the shallow nearshore to depths of thousands of meters are increasingly accessible; studies of interactions between specific organisms and physical and biological components are possible. The equipment used for sampling is dependent on the research questions asked and the characteristics and depth of the studied organisms and their habitat. Gears range from simple tools that are useful in shallow nearshore areas, such as bottles, secchi discs, and gillnets or beach seines to advanced equipment, such as remotely operated vehicles (ROVs), fishing trawls, and hydroacoustics deployed from large research ships for studies offshore and at greater depths. Even remote observation from space can be performed using satellites.

A characteristic transect from a continental landmass to the deep ocean includes nearshore environments that, depending on local geology, may consist of sandy beaches, cliffs or fjord systems. The continental shelf may stretch out some distance from the continental landmasses, gradually giving way to the continental slope, which descends down to the abyssal plains of the world's major oceans. As an example, the western coast of Norway contains an elaborate fjord system with numerous deep basins divided by shallower sills, giving way to the Norwegian Channel and then the shallower continental shelf. To the southwest, the North Sea is a shallow sea on top of a continental shelf only, while to the northwest, the Norwegian Sea descends into a deep-sea basin which also contains the Mid-Arctic Ridge, separating the Eurasian and North American continental plates. Banks, seamounts and submarine canyons are features that add to the topographical complexity of this general system (Figure 1.1).

Figure 1.1 Topographic chart of the North Sea.

Source: G. Macaulay, Institute of Marine Research, Norway.

Species composition changes with depth and distance from the coast, both for pelagic species and for organisms associated with the seafloor. Organisms are morphologically, physiologically, and behaviorally adapted to their environment through natural selection. Individuals with favorable genetic traits have increased breeding success than those lacking these traits (genetic adaptation). Some species are able to shift between environments and habitats, for instance benthic species with a pelagic egg and larval phase, or species that shift diurnally between different water depths (diel vertical migration, DVM). Diel vertical migration typically occurs between water masses with different properties in terms of light, temperature, oxygen, and salinity, requiring a physiological response from the organism. In general, effects of abiotic and biotic factors influence morphology, physiology, and behavior and thus how animals adapt to their habitat.

Examples of abiotic factors are the optical properties of the water column and include: light and the amount of suspended particles, which are important for visual predation; temperature, which regulates physiology, metabolic processes, and swimming activity; salinity, which affects physiology and osmoregulation; oxygen levels, which regulate respiration and metabolism and can limit reproduction or growth at low levels; and depth, which regulates pressure and affects buoyancy of fish that use swim bladders to obtain neutral buoyancy. Stratification of water masses, which often is seasonally dependent, limits nutrient availability in upper strata (the photic zone, as well as oxygen concentration in the lower strata or in isolated basins. Eutrophication and closeness to urbanized regions will also affect the level of primary production and the depth range where visual feeding is possible.

Biotic factors influencing the structure of marine communities and ecosystems include prey availability, predation, competition, and parasitism, and are regulated by direct or indirect access to production from lower trophic levels. Trophic communities in shallow waters benefit from readily available photosynthetic primary production, however, such production may be limited by nutrient availability. Organisms in deeper layers usually depend on energy and biomass from above either through migrating animals, transporting nutrients from surface waters to depths, or through the downward transport of debris, dead organisms, and particulate organic matter (POM). Because lower systems are dependent upon the upper regions, total biomass often decreases with depth. Population and individual growth potential will be further regulated through food access and competition. Access to reproduction (mates and spawning grounds) and reproductive behavior (nest spawning, demersal spawning, or pelagic spawning) will affect recruitment to populations. Presence of suitable nursery environments (e.g., coral reefs and kelp zone habitats) regulates survival of early life stages (larval stages of benthos and juvenile fish). Mortality risk (predator density, visibility, and size) in the habitats also changes with depth and distance from the coast.

Chapter 1 begins with a brief description of zonation in the pelagic and benthic realms, followed by a description of the topographies of coastal and fjord biotopes, the continental shelf and slope, and the deep ocean. These biotopes shape the habitats for bottom associated marine organisms. This is followed by a description of the physical characteristics of the pelagic ecosystem, including circulation of water masses in fjord ecosystems and a description of the light environment in marine waters. The chapter ends with an overview of temperate organisms (benthos and fish) that inhabit the littoral, sublittoral, continental shelf and slope, deep fjords, and the deep sea.

1.1 Marine Habitats

1.1.1 The Pelagic and Benthic Realms

The oceans are commonly divided into the pelagic and benthic realms. The pelagic realm refers to the body of water from above the seabed to the surface of the water. The organisms swimming or floating in this water column are termed pelagic and can be roughly divided into nekton, able to control their position in the water masses, and plankton. Traditionally the pelagic realm is subdivided into five zones:

1. The epipelagic: from the surface to about 200 meters and where the amount of UV light from the sun still allows photosynthesis.
2. The mesopelagic zone: from about 200 to approximately 1000 meters. The twilight zone where the organism still might be able to detect sunlight, but at which depth the energy from UV light is too limited for photosynthesis.
3. The bathypelagic zone: from 1000 to about 4000 meters, where no sunlight remains.
4. The abyssopelagic zone: from about 4000 to 6000 meters. The average depth of the largest oceans in the world is largely contained in this zone - between the 3300 meters of the Atlantic Ocean to the 4300 meter average depth in the Pacific.
5. The hadopelagic zones: between 5000 to 6000 meters. These zones are found in relatively restricted areas like deep trenches to the deepest trench, the Mariana trench, which is about 11?000 meters deep.

The benthic realm is defined as the bottom sediment or seabed of the ocean and the organisms in or on it are defined as the benthos. Organisms living in the benthic realm are living in close a relationship with the sediment, often permanently attached to it (epibenthos) or burrowing in it (endobenthos), while others, although they can swim, are never found far away from the seafloor, on which they are totally dependent (hyperbenthos or, in the case of fishes, demersal).

About 80% of the ocean floor consists of soft sediment, which can be designated as marine sediments of particle size ranging from mud to coarse sand (0.05?mm to about 1?mm in diameter). This entails that this soft-bottom substrate type defines the vast majority of habitats, from the high subtidal zone to the deepest part of the abyssal zone. Obviously, the term is very broadly defined, and soft sediments can be divided into a number of subhabitats, which are dependent on latitude, temperature and other local environmental factors, including a wide size range and a high diversity of associated organisms.

Ocean hard bottom areas, while less extensive, represent important habitats distinguished by differences in biota composition and dominating life strategies compared to soft-bottom counterparts. Hard bottom seafloor is...