13.7: Chapter Summary

Special Characteristics of the Coastal Oceans.

Salinity and temperature are more variable, and turbidity is generally higher, in coastal waters than in the open ocean. River discharges often lower salinity and increase turbidity near river mouths, and a shallow halocline is often present in this region. Coastal currents are driven by local winds and are generally independent of ocean gyre currents, often flowing in the opposite direction. Benthos of the coastal oceans are more diverse and abundant than open-ocean benthos are.

Nutrient Supply to the Coastal Photic Zone.

The mixed layer in many coastal regions receives nutrients from river runoff, coastal upwelling, decomposition of organic matter on the shallow seafloor, eddies formed around shallow seafloor topographic features, and breaking internal waves. Nutrients tend to be retained in coastal waters because residence times of coastal water masses are often long. Coastal upwelling is most prevalent off the west coasts of continents where Ekman transport moves the surface layer offshore. Newly upwelled water may support only limited primary productivity until needed organic compounds are synthesized by flagellates. As upwelled water moves offshore, diatoms bloom and are consumed by zooplankton, and eventually nutrients are depleted and primary productivity declines.

Nutrients may be recycled as they are transported into subthermocline waters in fecal pellets, released there by decomposers, and transported back inshore and upwelled. The eggs or spores of many species are carried inshore in subthermocline water, are then upwelled, metamorphose or hatch, and move back offshore where phytoplankton food supply increases. There, they mature and produce eggs or spores that sink and reenter the circulation.

Seasonal Cycles.

In subpolar regions, primary productivity is very low, except during a short summer when light intensity is sufficient for photosynthesis. Nutrients are plentiful in areas where strong vertical mixing prevents a pycnocline from forming. In polar and some subpolar regions, vertical mixing is restricted by a halocline formed by freshwater runoff and sea-ice melt; hence, nutrients are limited and productivity is low.

In tropical regions, there is little seasonality and nutrients are always limiting, except in upwelling areas. However, coral reef ecosystems are highly productive because of nutrient recycling, primarily between corals and their symbiotic zooxanthellae.

In mid-latitudes, the winter water column is well mixed, and primary production is light-limited. In the spring, as light intensity increases, a plankton bloom occurs and continues until nutrients are depleted. Nutrient depletion is often accelerated by the formation of a shallow seasonal thermocline. Productivity remains low until the fall, when storms provide additional nutrients by vertical mixing and a limited fall phytoplankton bloom may occur. Nutrients are returned to the photic zone by winter mixing.

There is a succession of dominant phytoplankton species during seasonal cycles of middle and high latitudes. Generally, diatoms dominate when nutrients are abundant and are replaced by flagellates as nutrients decline or as silica is depleted. Species succession is similar each year, but it varies in timing and dominant species because of differences in climate and the physical and chemical characteristics of the ocean water. Juveniles of many animal species rely on particular phytoplankton species or types at particular times and locations. When the species succession is altered, and this food is unavailable, these species’ larval survival can be drastically reduced.

Algal Blooms and Dead Zones.

Intense blooms of dinoflagellates occur infrequently in coastal water, primarily at subtropical and middle latitudes. These episodic blooms can be natural. However, their frequency is increasing off many coasts, and evidence suggests that sewage-derived nutrients may favor the dominance of dinoflagellates over diatoms and contribute to some blooms. Certain dinoflagellates and diatoms produce substances toxic to vertebrates. Hence, blooms can kill fishes. They can also adversely affect human health, because the toxins do not harm shellfish but are toxic to people who eat the shellfish. The most prevalent dinoflagellate toxin causes paralytic shellfish poisoning in humans, so many valuable shellfishing areas are closed to harvesting during months when dinoflagellate blooms may occur.

When phytoplankton blooms collapse as a result of nutrient depletion, their decomposing remains can deplete oxygen in bottom waters, particularly where residence times are long and bottom waters are separated from surface waters by a seasonal thermocline. Anoxia can decimate benthic communities. The number of coastal areas experiencing periodic or permanent anoxia has grown from just a handful in the mid-20th century to many hundreds, and the number continues to grow. There is strong evidence that nutrients, especially nitrogen, in sewage is responsible for much of the increase in the frequency and geographic extent of hypoxia and anoxia in coastal waters adjacent to major cities.

Fisheries.

Fishes are most abundant in areas of high primary productivity, especially in coastal waters, where trophic efficiency is higher because food webs are shorter. Coastal upwelling regions account for about half of the world’s fishery production. Natural variations in fish stocks and reproductive success are so great that maximum sustainable yield cannot be established accurately. Many fisheries are currently exploited at or above their maximum sustainable yield, and a number have collapsed. Global fisheries production appears to have peaked and is now declining.

Estuaries.

Estuaries are regions where seawater mixes with freshwater from rivers. Each estuary is unique, so there is no simple description of a typical estuary, and no single classification system can capture the many variations. Most present-day estuaries were formed as sea level rose during the past 19,000 years. One way to classify estuaries is by their geologic setting. Coastal-plain estuaries are drowned river valleys, and are abundant on passive margins. Bar-built estuaries are located landward of sandbars or barrier islands. Tectonic estuaries are formed by uplift or subsidence of the coast at a fault. Fjords are drowned valleys previously cut by glaciers.

Another classification system categorizes estuaries by their dominant circulation pattern. In salt wedge estuaries, seawater forms a lower layer separated by a sharp halocline from a low-salinity surface layer. Seawater progressively mixes into the surface layer as this layer moves seaward, raising its salinity. In partially mixed estuaries, the halocline is much broader, usually because of greater vertical mixing by tidal currents. In well-mixed estuaries, the water column has vertically uniform salinity, but salinity progressively increases seaward. In all but inverse estuaries, residual (nontidal) currents due to the estuarine circulation flow landward in the bottom part of the water column and seaward in the surface layers. The flow rates of seawater into the estuary and of lower-salinity water into the ocean are many times greater than the flow rate of freshwater into the estuary. In the Northern Hemisphere, the Coriolis effect deflects flow into the estuary toward the right side as viewed from the ocean and deflects the outflow to the left side. Fjords have estuarine circulation only at depths above the top of their sill. Water below the sill depth is stagnant, often anoxic, and may be enriched in hydrogen sulfide. Occasional flushing of anoxic bottom water enriched in hydrogen sulfide can cause fish kills.

Particles and associated toxic metals and organics tend to be trapped and accumulate in estuaries. They can move seaward in the upper layer, but they sink to the lower layer that moves landward in estuarine circulation.

Estuaries are stressful environments for organisms because of variable temperature, salinity, turbidity, currents, and other environmental factors. However, they provide food and shelter from ocean predators, particularly in bordering wetlands. The estuarine circulation enables weakly swimming or planktonic species or their eggs and larvae to remain within the estuary. Anadromous fishes live in the ocean and transit estuaries to spawn in rivers. Catadromous fishes live in rivers and transit estuaries to spawn in the oceans.