At first, this seems like a no-brainer; if you add nutrients, you grow phytoplankton, which are food for fish. But growing fish in aquaculture is actually done a very different way. Carp, which is the major food fish grown by aquaculture in the world today, are produced by adding nitrogen and phosphorus to small ponds where carp is the only fish present and not surprisingly carp grows quite well. Catfish is another bottom feeding fish that is produced successfully through aquaculture, and also grows well under these conditions.

But what would happen if you added inorganic nutrients to real lakes with complex food webs? Would fish yields increase? The answer is that the success is somewhat mixed and limited, especially for target fish species. Sometimes you get fish, but you don't necessarily get the ones you wanted.

imageI had to look really hard to find these two studies that demonstrate that increase nutrient additions to lakes leads to increase fish production. Here is the application of fertilizer by seaplane to some very oligotrophic (i.e. low nutrient, low biomass) lakes in British Columbia. This is work by John Stockner and colleagues and the probably the closest study we have to oceanographic analogy, because these are really oligotrophic lakes.

imageThe scientists fertilized quite a few lakes, and here are a few variables before and after fertilization, and the percent change. So after fertilization, we've double the chlorophyll concentration, which is a measure of phytoplankton growth. That is a pretty impressive result. We also saw a big increase in zooplankton, which is essentially the food for small fish. There was a measurable increase in younger fish, and one-year-old salmonoids, but we had no data on adult fish. So can we increase fish production in British Columbia lakes by adding nutrients? Maybe is how I would characterize that work. At least we can increase the production of young fish. It was not evident that you could increase the production of adult fish.


imageThis is the second experiment I could find. This is actually a picture of Toolik Lake, which is a long-term study site in Alaska. Lake N1 was fertilized for several years at four times ambient level in an experiment. The question was could you increase fish production in this system?

imageIn Lake N1, there's really no effect on the weight of fish until the end, and there is an increase in the number and intensity of fish after fertilization. Lake N2, where they tried a similar experiment, was a total failure and I wont show the data. There was no evidence that fish yields increased.

imageSo to answer the question of whether fertilization increases fish production, there have been as many failures as successes in the published literature. One can only wonder about the number of studies that were never published. It's really hard to collect the necessary data; it is very difficult work to do, and subsequently finding data on the effects with adult-size fish are really rare.

imageSo to sum up, I think that lake experiments are useful for identifying some processes that might be relevant for larger scale marine experiments. The lake results, to date, suggest that the effects of nutrient fertilization on CO2 are not simple to predict. Much of the initial CO2 uptake by phytoplankton is eventually respired by bacteria during decomposition over a time-scale of months. We were unable to stimulate the CO2 sink in the presence of piscivore fish, thereby demonstrating that the food web matters a great deal to CO2 dynamics. So it's not at all a sure thing that ocean fertilization will lead to atmospheric CO2 drawdown, and it's not a sure thing that ocean fertilization will lead to increased fish production on a large scale.

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