9.1 Ocean basics

Salinity and temperature

Earth’s major oceans are a series of linked basins interrupted in places by continents that can only manage to amount to about one-quarter of the planet’s surface. When Earth was young, the oceans were much less salty, but a few billion years of rain washing dissolved minerals out of the rocks have brought it to its present salinity of approximately 35 parts per thousand, about 6 times saltier than our blood.

Salinity, like many marine water characteristics, varies more than one might expect in a well-mixed system. But some parts of the oceans are less well mixed. In very shallow areas, evaporation may increase salinity. Where ice is melting and running into the oceans, and where rivers meet the oceans, salinity will be lower.

Ocean water is colder at depths than near the surface. In part, this is due to solar warming, which obviously occurs only at the surface.  As well, cold water is dense, and sinks, ensuring that deeper waters are always colder. Mixing by wind in shallow waters and by currents throughout much of the oceans modifies the temperature profile. In areas where cold water is brought to the surface by currents meeting obstacles or by wind moving surface waters out of the way and providing an opening for rising water, surface waters can be quite cold (Fig 1). These cold-water upwellings are places of high productivity and are often biodiversity hotspots as well as fishing hotspots. Except for cold-water upwellings, sea-surface temperatures largely vary by latitude, but the depths are uniformly cold, except for a few hot-water vents.

Salinity and water temperature, together, create the global circulation pattern among ocean waters that is called the Meridional Overturning Circulation (Fig 2), which we saw briefly in one of the Chapter 1 videos. Warm water in the tropics moves polewards (north in the Northern Hemisphere, south in the Southern Hemisphere) because of the Coriolis effect, carrying warm, salty water away from the equator, where evaporation slightly increases salinity. In the Atlantic, the Gulf Stream carries the warm water from the Gulf of Mexico clockwise along the North American East Coast and across the Atlantic towards the United Kingdom. The water cools as it travels, and as it nears Greenland, that cooling, dense, salty water sinks towards the ocean bottom. The cold water travels along the ocean floor, south through the Atlantic Ocean, until it meets the circumpolar current of the South Ocean, and joins it. Because of wind patterns in the South Ocean that blow towards the equator, water is lifted up from the deep Southern Ocean at several points, warming as it reaches the surface, and returning towards the Atlantic.

Because the driving part of the circulation pattern, the sinking cold, salty water, is in the North Atlantic, the pattern is often called the Atlantic Meridional Overturning Circulation (the AMOC),  although it affects all the main oceans of the world. The branch of the AMOC that comes from the Gulf of Mexico towards Europe is responsible for milder temperatures in Europe than would otherwise occur at that latitude.

Dissolved gases

As always, gases dissolve best in cold water. Although plants take carbon dioxide from the air, phytoplankton use dissolved CO₂, which dissolves readily in water and is not limiting. Under climate change, levels of dissolved CO₂ are increasing as CO₂ in the atmosphere increases.

Fish and other marine organisms rely on dissolved oxygen, which is usually not limiting. However, the ocean has dead zones, often as the result of anthropogenic eutrophication, as we saw in Chapter 3. Some eutrophic areas occur naturally, in upwelling areas where nutrients come to the surface in abundance. Where eutrophic areas create large algal blooms, and dead algae feed decomposer bacteria that use up oxygen in the water, oxygen can be limiting, as we saw in the discussion of dead zones in Chapter 3.

Minerals

Ocean water contains many dissolved minerals, most in very small quantities. Gold, for example, is present in parts per trillion – levels that are currently not useful for recovery.

Over long periods of time, some minerals precipitate out into mineral deposits on the ocean floor as a result of chemical reactions that create insoluble forms. Mineral deposits may occur in crusts, nodules, and large ore bodies. Metals and critical minerals are of particular interest to industry.

Biodiversity and ecology

Owing to their vastness and the difficulty researchers face in studying oceans, more than 90% of ocean species may still be uncatalogued, and some 80% of the oceans are unmapped.^[1]

As in lakes, productivity is related most to light availability and nutrient availability. As a result, coastal areas are often productive because shallower water and nutrients from runoff supply nutrients that surface-water organisms such as phytoplankton (the base of the food web) can access. In contrast, in deeper parts of the ocean, the surface waters have sunlight, but nutrients are much less available, except where upwellings bring them to the surface from the depths.

Phytoplankton – algae and blue-green algae (actually bacteria) produce approximately 50% of the world’s oxygen. The tiniest species of these produces some 20% of world oxygen, outstripping all the world’s tropical rainforests.^[2]

Carbon and nutrient cycles in the ocean are somewhat complex. Gravity wins a lot of battles, and many minerals and a lot of carbon end up on the sea floor. But in the water column, organisms capture other living and dying organisms, eat fecal pellets or dead material, and keep some carbon and nutrients in constant circulation in the upper parts of the water column (Fig 3). We are still learning about oceanic circulation in important ways, particularly as concerns about climate change lead us to understand carbon cycling better.

Only recently, for example, researchers realized that, before humans hunted whales to near extinction, whale urine and feces and whale bodies were an important source of carbon to the sea floor and to the organisms that inhabit those depths. The whales that eat plankton feed in cold, productive waters and then travel to warm, nutrient-poor waters to give birth, transporting nutrients in their urine, feces, and bodies, and improving productivity in these waters, in much the same way that large herbivores move resources around grasslands. And just by existing, whales can, over their lifetimes, store more carbon than long-lived trees.^[3] As some whale populations are recovering, it is becoming easier to understand these mechanism. The overall movement of carbon from the atmosphere into the oceans is called the carbon pump. It is an important mechanism of climate regulation. The whale portion of it has been referred to as the whale pump.

Ecosystem services of oceans

Provisioning services of oceans provide food, raw materials for building and manufacturing, medicines, and minerals. Although aquaculture production is increasing, capture fisheries – fish and other marine organisms taken wild from the waters of the world –  still comprise about half of aquatic food production, and about 90% of capture fisheries are oceanic. Coral and sand are used for construction, with sand (necessary for cement and concrete production) increasingly taken not only from beaches but also sucked off the ocean floor along with any living things inhabiting it. Marine organisms contribute to cosmetics, dyes, food additives, fertilizers, paints, abrasives, and many other kinds of products. Marine products are used for medicines including anti-cancer drugs and painkiller, among others. Minerals are not yet widely produced from ocean sources, but as technological demand for them increases, the availability of ocean sources for metals and other critical minerals is expected to result in mining of the sea floor. Oceans also provide a medium for transportation. An estimated 80% of international trade moves by sea.

Among regulatory services, climate regulation is perhaps the ocean ecosystem service most under study. Approximately 90% of anthropogenic heat generated through climate change has been absorbed by the oceans, so far,^[4] and about 30% of anthropogenic CO₂.^[5] Without these services, the planet would have warmed much faster than it has to date. Additional regulatory services include protection of coasts, which is afforded by coral reefs, mangrove forests, and salt marshes, which break the force of storm surges, and dilution and breakdown of water pollutants.

Supporting services provided by oceans include oxygen production which supports most life forms, primary production via photosynthesis, which provides the basis for the food webs of the ocean, and the provision of habitat for marine organisms. These are quick to describe, but enormous in their impact.

Life on Earth began in oceans and although the land now supports the majority of biomass and biodiversity, oceans are crucial to providing a sustainable environment for all life.

Governance of the oceans

The overarching statute governing human interactions with oceans is the UN Convention on the Law of the Sea, sometimes abbreviated UNCLOS. Most nations of the world, but not all, have ratified the treaty. The US has not ratified it, but abides by its provisions, to date. UNCLOS sets up a variety of frameworks for dealing with ocean issues. It defines national territorial waters as extending outward from national seashores for 12 nautical miles (13.8 statute miles, 22.2 km). The waters within that distance are part of the sovereign territories of the respective nations and they may make and enforce laws governing any aspects of these waters without interference from other nations. Further, nations have sovereign rights over natural resources such as fish and minerals within 200 nautical miles (230 statute miles, 370 km), regions known as exclusive economic zones or EEZs. Beyond territorial waters (for matters not related to natural resources) or the EEZs (for matters related to natural resources), issues of navigation, resources management and use, environmental protection, research, and settlement of disputes about these are governed by UNCLOS, for those nations that have ratified the treaty.

UNCLOS also sets up a means for additional agreements to be created to deal with specific marine issues. For example, the International Seabed Authority was created to address seabed mining. The UN Fish Stocks Agreement, which manages stocks of fish in international waters and stocks that cross international boundaries, also arose out of the UNCLOS framework.

Early overharvest of marine resources such as whaling and harvesting of seabirds illustrated clearly the potential for tragedies of the marine commons. UNCLOS and the treaties and agreements that grew out of it are the major means of converting high seas resources from common-pool resources that can be freely (and excessively) exploited by all into resources subject to sustainable management.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.