In terms of the demand for CO2(aq), an enlarged endosymbiont population increases the likelihood of CO2(aq) becoming a limiting internal substrate during periods of peak photosynthesis [18, 19]. Several environmental factors favour increased zooxanthellae densities (particularly on a per host cell basis), including: (i) elevated nutrient levels (e.g. dissolved inorganic nitrogen, DIN) in the surrounding sea water , elevation of the CO2 partial pressure (pCO2) in the surrounding sea water , and diffusive (i.e. branching) coral colony morphologies . Experimental manipulations confirm the higher expulsion rate of zooxanthellae during periods of high irradiance in branching corals  and in corals exposed to DIN and pCO2 enrichment [24, 25].
Association for the Sciences of Limnology and Oceanography
Table of Contents, Vol. 38 (2), 1993
NYGAARD, KARI, AND AUGUST TOBIESEN
“Bacterivory in algae: A survival strategy during nutrient limitation” p.273-279
Bacteria have a high P content even when phosphate is limited (Andersen et al. 1986), although their P content can be reduced when they are drastically starved (Tezuka 1990). Bacteria are therefore a potential source of P when obligate phototrophic algal flagellates are subjected to P limitation, because bacteria are more efficient at sequestering P under these conditions (Bratbak and Thingstad 198 5).
Phosphate deficiency alone may be insufficient for the success of bacterivorous algae because their growth requires that other environmental factors are also favorable.
Bacteria have been shown to out-compete algal flagellates in phosphate-limited chemostats at low dilution rates (Currie and Kalff 1984; Bratbak and Thingstad 1985).
Our results suggest that algae able to graze and use P compounds in the bacteria will have a compensatory mechanism to overcome their competitive disadvantage.
If you want to avoid toxic algae in your tank, avoid this condition!
Bernhard Riegl, Carlton Heine, and George M. Branch. 1996. Function of Funnel-Shaped Coral Growth in a High-Sedimentation Environment .Marine Ecology Progress Series : 87 -93.
“Current speeds between 30 and 90 cm s-1 were enough to clean the funnels of 3 experimental grain sizes (coarse, fine, medium sand).”
Dubinsky Z, Jokiel PL. 1994. Ratio of energy and nutrient fluxes regulates symbiosis between zooxanthellae and corals. Pac Sci 48(3): 313-324.
Full article available.
[Summarizing] the main interactions and feedback mechanisms connecting light intensity, nutrient level, and feeding in zooxanthellate corals:
- Under constant nutrient concentration, light intensity determines the onset of nutrient limitation; as light increases, C : N ratios exceed Redfield ratios.
- The availability of other nutrients, mainly nitrogen, determines the fate of photo-assimilated carbon. Under high C: N ratios, most carbon goes into respiration, calcification, and excreted mucus, whereas low C : N ratios favor increases in zooxanthellae density, reduce translocation, and slow down calcification.
- Feeding on zooplankton by the coral under low light provides carbon for metabolism. Under high light it supplies both algae and animal with nitrogen.
Nutrient balance is one of the most important, but seemingly lease understood aspects of aquarium keeping. This statement from the article summarizes it as well as I’ve seen it summarized. I’ve edited only for clarity.
Also from this article:
Nutrients acquired by predation of the coral on zooplankton are available first to the animal, whereas those absorbed by the zooxanthellae from seawater as inorganic compounds lead first to growth of the algae.