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.
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]
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. ]