Under controlled laboratory conditions the influence of increased water fertility was tested in four spinose planktonic foraminifers. Among other things, increased water fertility may be characterized by high prey densities and longer wavelength, low light conditions. Normally, a larger test is formed at higher feeding rates. This is shown most distinctly in Globigerinella siphonifera (d'Orbigny). The effect of increased food availability on test size of Orbulina universa d'Orbigny is larger at lower temperatures, a condition that prevails during upwelling events. For logistical reasons, the mean final size of Globigerinoides sacculifer (Brady) does not show a consistent tendency with feeding rate. Globigerinoides ruber (d`Orbigny) does not tolerate high levels of brine shrimp and was thus not kept at higher feeding levels.Globigerinella siphonifera and O. universa reach smaller shell sizes under light conditions that characterize fertile regions (yellow-green) relative to typical open ocean light conditions (blue). In contrast, G. ruber attains smaller shell sizes under blue light relative to yellow-green light. Again, G. sacculifer does not show a clear trend with respect to final test size under normal and upwelling light.The effect of longer wavelength light counteracts and overrules the effect of increased prey capture rates in the investigated symbiont bearing species. This phenomenon is explained as photosynthesis-limited host respiration. The extent to which the light effect counteracts the effect of feeding probably depends on habitat to which the species are originally adapted. Qualitative and qualitative pigment analyses suggest that G. ruber lives shallow, G. sacculifer lives deeper and that G. siphonifera has the deepest habitat. Data from the literature indicate that O. universa lives at intermediate depth between G. siphonifera and G. sacculifer. As G. siphonifera is somehow adapted to low intensity longer wavelength light, the counter effect of low intensity and longer wavelength light is minimized in this species. As a result, the effect of increased feeding rate is most pronounced in this species.Fertile water is found in upwelling settings and in areas where rivers enter into the ocean. Comparison of sediments from fertile regions and neighboring open ocean sites shows that symbiont bearing planktonic foraminifers attain smaller shell sizes in the upwelling area off Benguela but that larger shells are found in the Zaire river plume. Due to the small quantity of reference sediment, statistically significant differences were found only for O. universa. The different response is explained as the result of limited respiration in upwelling areas because of lower than usual oxygen levels.The morphology of the final chamber is influenced by both light quality and feeding rate. At higher feeding rates the number of sac-like phenotypes is decreased. Yellow-green light induces more kummerform morphologies in G. sacculifer, G. ruber, and G. siphonifera and increases second sphere formation in O. universa. Light quality effects the terminal morphology to a larger extent than prey capture rates do. Unfortunately, these trends could not be confirmed in our reference sediment due to the low number of kummerforms and sac-like chambers.Water fertility also influences the fractionation of stable oxygen isotopes. It is concluded that G. sacculifer does not secrete carbonate in equilibrium with ambient seawater. Isotopically lighter tests are constructed under longer wavelength light conditions and they tend to get heavier with increasing prey capture rates. Specimens with sac-like chambers are lighter than normalform specimens whereas kummerform types are heavier than normalform types.