# The effect of depth on net primary production in aquatic ecosystems

The figure shows the relationship between the water depth and net primary production (=P-R). I want to know why the production (P) initially increases with water depth near the surface? I have seen similar relationships from other sources but never seen a clear explanation.

• Is the figure referring to gross primary production or primary production per capita? Dec 13 '16 at 16:27
• Textbooks don't give details. At first, I thought the pattern of the productivity correlates with the abundance of phytoplankton. But because the rate of respiration is constant, now I think it is the per capita effect. If it is not the per capita effect, I like to know why the total respiration is constant. Dec 14 '16 at 1:28
• But the texts in the figure (and other similar figuers from different sources) imply that the figure shows NPP = P - R. So I think it is probably not the per-capita effect. But this figure (jochemnet.de/fiu/Ink1.jpg) shows that the pattern does not emerge from difference in the concentration of phytoplankton. Dec 14 '16 at 2:47
• Ok, a bit hard to answer if it is unclear whether the effect is per capita or the total productivity. For experts in the field I suspect it is obvious though (I'm not an aquatic ecologist). My initial guess was however that the graph showed total primary productivity (for the entire "ecosystem"), and that the dip towards the surface is related to the density of primary producers. Lower density at the surface could then be related to both turbulence (waves) and predator avoidance (among other things). Dec 14 '16 at 9:49
• It looks like the result like the one shown in the figure is based on a controlled experiment using the dark/like bottle method (jochemnet.de/fiu/Ink1.jpg). Because the density of phytoplankton is controlled in each bottle, we can qualitatively interpret the result as the per-capita effect too. Because of this, the effect should be entirely based on physical factors (e.g., there are no predators in the bottles). Turbulence can still influence the bottles, though. Dec 14 '16 at 10:32

After a quick glance at the book "Light and Photosynthesis in Aquatic Ecosystems" by Kirk (2010), I think that the cause for the productivity dip towards the surface partially lies in photoinhibition, due to high light intensities at the surface. Here are a couple of relevant quotes from the book (Google books: p. 371):

In this light saturated state, the electron transport and/or CO2 fixing enzymes (most likely, the latter) are working as fast as they are capable, and so any additional absorbed quanta are not used for photosynthesis at all. From the end of the linear region through to the light-saturated region ([i.e. close to the surface, my addition]), since photosynthetic rate does not increase in proportion to irradiance, (P/Ed steadily falls, see Fig. 10.3) the quantum yield and conversion efficiency necessarily undergoes a progressive fall in value. This is accentuated further if, at even higher light intensities, photoinhibition sets in. If the cells contain photoprotective carotenoids, in which absorbed light energy is dissipated as heat rather than being transferred to the reaction centre...

However, the suspended bottle methods often used to estimate these depth gradients might be part of the problem, by overestimating the effect of photoinhibition, by forcing plancton to stay at the same depth (p 358):

Depth profiles of phytoplancton photosynthesis, such as those in Fig. 10.4, determined by the suspended bottle method, tend to overestimate the extent to which photoinhibition diminished primary production. In nature, the phytoplancton are not forced to remain at the same depth for prolonged periods. Some, such as dinoflagellates and blue-green algae, can migrate to a depth where light intensity is more suitable. Even non-motile algae will only remain at the same depth for extended periods under rather still conditions.

I hope I got the quotes right (quick manual retyping). There are many more relevant sections in the book as well, which seems to cover all kinds of aspects of aquatic photosynthetic efficiency and how this can be a function of depth.

I think it has to do with what wavelength of light is absorbed by photosynthetic organisms at what depth.

Ultraviolet light with short wavelength is absorbed closest to the surface. Red light (which is responsible for photosynthesis) is absorbed at a deeper point in aquatic systems by the primary producers like phytoplankton and metaphyta which increase the productivity of that particular depth.

[I'll include citations, references and diagrams as soon as I find enough time]

• The amount of red light monotonically decreases with the depth. So it still does not explain the hump shaped productivity pattern. Dec 13 '16 at 2:11

Compared to the surface, slightly below the surface in the water column the availability of nutrients increases as winds and ocean currents cause increased mixing of nutrient rich deep water. The photic zone of the water column is quick to use up the macronutrients necessary to sustain primary production, however upwelling of nutrient rich deep water becomes a significant factor in determining an area's production.

[ I will provide citation later. ]

• But according to this figure (jochemnet.de/fiu/Ink1.jpg), the result is based on controlled experiments (the dark/light bottle method). Therefore, the available nutrients are the same along the depth (even if they are naturally variable). Dec 14 '16 at 8:05
• If this is the experiment I believe it is, then the bottles filled at different depths and returned to the same depth they were filled at. This would have a difference in available macronutrients. However this article about the experiment also details that primary production increases just deeper than the surface because "Primary production is usually suppressed just at the surface because the light is too strong there, but reaches a maximum just below the surface because this is where the light optimum occurs. Below the light optimum, production declines rapidly with light." Dec 14 '16 at 15:12
• Article location: www-personal.umich.edu/~gwk/teaching/limno/lab/onlinemanual/… Dec 14 '16 at 15:13