A number of reasons.

The atmosphere-ocean exchanges variability are not only due to biology, but also due to thermodynamics. Basically colder water can hold more CO2. So the seasonal cycle of ocean-atmos. exchange is controlled not only by growth of marine plants but by seasonality of temperature, mixed-layer depth and other dynamic variables.

The seasonality of o-a exchange is not as simple to characterise in terms of seasonality because the Southern Ocean plays such a big role and of course its seasonality is in the opposite sense to the Northern Hemi. Some of the most important areas of o-a exchange are modulated by by processes like upwelling which do not correspond clearly to either N or S Hemi. seasonality. Examples include monsoonal upwelling zones in the Arabian Sea.

Also biomass in the ocean is labile (short-lived). That is, it’s turned over much more than terrestrial biomass. Even though ocean productivity plays a big role in the global carbon cycle, marine biomass (or standing stock, or inventory) is orders of magnitude smaller than terrestrial biomass. There are no “trees” in the ocean and its ecosystems are dominated by planktonic micro-algae whose lifespans are measured in weeks or months (even the dramatic kelp “forests” off Tasmania and the US West Coast do not account for very much biomass). Proposals like iron fertilization that seek to stimulate ocean plant growth would not work by storing carbon as biomass, but by storing it as dissolved carbon dioxide (mainly bicarbonate ion) in the deep ocean. The carbon would be transferred to the deep ocean as biomass, but it would be quickly converted to dissolved CO2 by decomposition. So it’s a somewhat different system.

And thanks for the clarification of Dyson’s seasonal flux comments. I interpreted his description in the same way as you outlined. I think what Dyson was after was to get a handle on the rate at which carbon could theoretically be removed from the atmosphere by some future geo-engineering approach. He wrote:

This fact, that the exchange of carbon between atmosphere and vegetation is rapid, is of fundamental importance to the long-range future of global warming, as will become clear in what follows. Neither of the books under review mentions it.

In this back-of-the-envelope estimation of the first derivative Dyson ignored the seasonal atmosphere-ocean exchanges. Why?

http://www.grida.no/climate/vital/13.htm

for an illustration of the reservoirs and fluxes in the carbon cycle.

The annual cycle Dyson cites is mainly a “deciduous” signal: net ecosystem respiration (due in part to decaying leaves) versus net ecosystem photosynthesis. Only net growth of tree trunks and branches, or carbon buried in soils or peats “counts” as sequestration in this cycle.

So any biotech solution would have to be a “tree-trunk” solution.

see:

Canadell, J. G., Le Quere, C., Raupach, M. R., Field, C. B., Buitenhuis, E. T., Ciais, P., Conway, T. J., Gillett, N. P., Houghton, R. A., and Marland, G., 2007, Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks: Proceedings of the National Academy of Sciences, p. 0702737104, doi:10.1073/pnas.0702737104.

and

http://www.globalcarbonproject.org/carbontrends/index.htm