12.5: Phototropic Primary Production and Light

Seawater absorbs sunlight, so no light at all reaches ocean depths greater than a few hundred meters, even in the clearest open-ocean waters (Chap. 5). In coastal waters with high concentrations of suspended sediments, turbidity is high and light is more effectively absorbed and scattered. Turbidity is generally highest in shallow coastal waters where bottom sediments are resuspended by waves and in estuaries fed by rivers with high suspended sediment loads. In such waters, light penetrates a few meters at most, and only centimeters in extreme cases.

Phototrophy, including photosynthesis, cannot take place without light. Light is absorbed by the water, and its intensity is reduced with depth. Light of sufficient intensity to support photosynthesis, usually thought to be about 1% of the intensity at the surface, rarely penetrates more than about 100 m, and photosynthesis is generally restricted to the water column above this depth. This upper layer is called the photic zone.

Terrestrial plants must obtain nutrients from soil or, in some cases, from raindrops. In contrast, marine organisms can extract nutrients from the surrounding seawater. As a result, they do not have to be attached or rooted to the seafloor. Furthermore, attached marine photosynthesizers can receive enough light for photosynthesis only if they are attached to the limited areas of seafloor that are shallower than the depth of the photic zone. Consequently, most marine photosynthetic species live in the water column and are not attached to the seafloor.

Because they need light and therefore must remain near the surface, even where the water is deep, most marine phototrophs must avoid sinking. Pockets of gas or air contained within some species of algae enable them to float. The bubblelike lumps on some seaweeds (Fig. 12-4a) are such pockets. Some species that live attached to the seafloor also use these devices to keep their upper fronds in the light, including the giant kelp in California coastal waters (Fig. 12-4b).

   

Figure 12-4. Many species of macroalgae have gas enclosures within their structure that enable them to float and stay in the surface waters where there is ample light for photosynthesis. (a) The round ball-like structures in this kelp frond (Macrocystis sp.) are gas-filled. (b) Masses of kelp (Macrocystis sp.) grow in this cove near Monterey, California. The plants are anchored to the seafloor, but you can see the upper ends of the kelp fronds covering much of the water surface, held up by their gas enclosures.

Phytoplankton include algae and many species of bacteria and archaea that are autotrophs (either phototrophic or chemosynthetic). Phytoplankton are the major primary producers in the oceans and are abundant especially in productive ocean waters. Most phytoplankton species are microscopic and drift freely in the photic zone. Due their small size and the relatively high viscosity of water (Chap. 5), phytoplankton cells sink very slowly (CC4), even if their density is higher than that of seawater, and this helps them to remain in the photic zone. As discussed elsewhere in this chapter, the microscopic size and, therefore large ratio of surface area to volume, of most phytoplankton species also aids them in taking up dissolved substances. 

In addition to sinking very slowly due to their small size, turbulence due to waves and currents counteracts the tendency of small phytoplankton to sink (CC4). Some species of diatoms, which are among the largest phytoplankton, store oils that increase their buoyancy, and many also have spines or aggregate in chains, which also reduces their tendency to sink because this increases drag. Dinoflagellates, another group of phytoplankton species, are able to counteract sinking because they have whiplike flagella that provide them with limited motility.

Phototrophy, Light, and Depth 

Until near the end of the 20th century, it was thought that almost all ocean life ultimately depends on photosynthetic primary production for food. Other food sources, other phototrophic and chemosynthetic primary production (especially in hydrothermal vent communities) and the dissolved or particulate organic matter supplied by rivers, were previously considered insignificant in comparison. Research in hydrothermal vent areas and studies in estuarine and coastal waters have shown that these sources are probably much more important than previously thought. Because the relative importance of these other sources is not yet well known, we will focus heavily on photosynthetic primary production in this chapter.

Because photosynthesis requires light energy, most of the primary production in the oceans takes place in the upper layer of the water column, down to the depth that sunlight penetrates. Light intensity at any given depth is determined by the intensity of sunlight reaching the sea surface, the angle of incidence of the sun’s light to the sea surface, and the water turbidity. The intensity of sunlight reaching the sea surface varies with the seasons, throughout the day as the angle of incidence of the sun changes, and as cloud cover varies. Light intensity at a given depth changes continuously. Fortunately, at any given location and depth, the variation of daily mean light intensity within a season is relatively small in comparison with the variation between seasons. Hence, the daily average intensity is a reasonable measure of the amount of light available for photosynthesis at any location and depth.

Light intensity decreases rapidly with depth (Fig. 5-18). In the clearest ocean waters, light intensity is reduced to 1% of the surface intensity at a depth of about 100 m. In coastal-ocean waters, the corresponding depth is variable, but it is often about 5 to 10 m. The rate at which photosynthesis can occur is reduced as light intensity decreases (Fig. 12-5). In contrast, the rate of respiration remains virtually constant at all depths because the energy needed to fuel the life processes of all species including photosynthetic autotrophs do not change significantly with depth. At relatively shallow depths, the rate of photosynthesis exceeds the rate of respiration, and photosynthetic autotrophs can grow larger and reproduce. At depths where the respiration rate exceeds the photosynthetic rate, autotrophs must metabolize more organic matter than they create by photosynthesis in order to survive. At least some species of photosynthetic autotrophs can survive for a limited time at these depths, but they cannot grow or reproduce. Although many species die if they do not return to conditions in which photosynthesis exceeds respiration, some species are capable of entering a resting phase, much as terrestrial plant seeds do, and can survive prolonged periods below the photic zone. Some microbial photosynthesizers are mixotrophs, meaning they are capable of switching from autotrophy to heterotrophy when insufficient light energy is available. 

Figure 12-5. Photosynthesis depends on the availability of light energy. Therefore, the rate of primary production by phytoplankton decreases with decreasing light intensity and, thus, with depth. The rate of metabolic energy use and, thus, respiration by phytoplankton remains almost constant regardless of depth. The depth at which the rate of phytoplankton primary production equals the rate of phytoplankton respiration is called the “compensation depth.” Below this depth, phytoplankton use more organic matter for respiration than they can produce by photosynthesis. The water column above the compensation depth is the photic zone, and everything below this depth is in the aphotic zone.

The depth at which photosynthesis equals respiration is called the compensation depth. The water column between the ocean surface and the compensation depth is the photic zone, and depths below this region constitute the aphotic zone. The compensation depth identifies the depth range within which phytoplankton and other photosynthesizers can produce more organic carbon than they consume. The compensation depth lies approximately where light intensity is reduced to 1% of that at the surface. Hence, the photic zone is rarely deeper than 100 m and is much shallower in turbid coastal waters (Fig. 5-18). Except in tropical regions, where the day length does not vary much, the compensation depth generally increases during local summer because the angle of incidence of the sun is smaller and the days are longer. However, in some regions where phytoplankton reproduce more rapidly, the additional phytoplankton cells and increased numbers of zooplankton that eat them contribute additional “particles” that absorb and scatter light. Consequently, in these regions the plankton reduce light penetration and the compensation depth, offsetting the effect of greater light intensity in summer.

Primary Production and the Ozone Hole 

Although the rate of photosynthesis usually decreases with increasing depth (Fig. 12-5), in many areas it is often lower in water immediately below the surface than at a depth of several meters. One reason for the lower photosynthetic rate near the surface is that high light intensity, especially ultraviolet light, appears to interfere with photosynthesis. Because ultraviolet light is absorbed rapidly in the first few meters of seawater (Chap. 5) the maximum photosynthesis often occurs slightly below the surface.

Chapter 7 discussed depletion of the Earth’s ozone layer by synthetic chemicals that may cause an increase in ultraviolet light intensity at the Earth’s surface. Higher ultraviolet light intensity may significantly increase near-surface inhibition of photosynthesis, reduce the total food supply, and adversely affect ocean ecosystems.