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I recently received a paper, written by some of my colleagues, who are
trying to predict phytoplankton blooms in human-impacted estuarine
areas, and I think their results can shed some light on our problems.
The citation is:
Roelke, D., L. Cifuentes and P. Eldridge. 1997. Nutrient and
phytoplankton dynamics in a sewage-impacted Gulf coast estuary: a field
test of the PEG-model and equilibrium resource competition theory.
I will paraphrase the main points from their paper:
Generally speaking, when nutrients are limiting, diatoms are superior
competitors for P, while green algae are intermediate and cyanobacteria
are poor P competitors. With respect to N, cyanobacteria are the
superior competitors, followed by diatoms and green algae. As a
consequence, when the N:P ratio is high (generally, 16 N atoms for every
P atom or more) and Si is available, diatoms are favored. When N:P is
high and Si is scarce, green algae are favored, and when N:P is low,
blue-green algae are favored (blue-greens can fix atmospheric nitrogen,
so they can exist in nitrogen-scarce environments). However, when both
N and P are plentiful, and light intensities are high, green algae are
favored because of their faster growth rate.
Algal blooms in nature are usually short-lived, on the order of a month
or two, typically because of the combined effects of nutrient depletion
and "grazing" by planktivores. Blooms are often successional, i.e., a
green algae bloom can be followed by a blue-green algae bloom, if N
becomes depleted and P is still abundant. This would certainly be true
in a low N:P environment. Blue-greens can become pervasive, as they are
not heavily grazed.
Anyway, these researchers followed several algal blooms through an
estuary, measured N and P over time, and documented the species
succession. In their conclusions, they stated "succession within the
phytoplankton community showed a degree of predictability to nutrient
perturbations. Therefore, management of the phytoplankton community
composition may be possible". Their ultimate goal is to prevent noxious
algal blooms (e.g., dinoflagellates and blue-greens) by nutrient
manipulation, while encouraging the growth of more desirable algae that
can be consumed by planktivores. This would provide benefits all the
way up the food chain.
In an aquarium, we can more easily manipulate nutrients and control the
effects of grazing than in the wild, so we have a definite advantage.
Increasing N and/or reducing P could be an effective control of green
water, and would also help prevent a successional blue-green algal
bloom. We can also add grazers--many people have had success with
Daphnia, and a ultrafine filter would also be an effective "grazer". We
can also "graze" a blue-green algal bloom by vacuuming. Water changes
would only be effective if the changing water has less N and P than the
aquarium water. Otherwise, a water change may actually extend the life
of the bloom.
Hope everyone finds this as interesting as I did. None of the authors
are aquarists, BTW.