In field experiments designed to induce dense phytoplankton crops by phosphate and nitrate additions to enclosures in a bed of Potamogeton filiformis in Loch Fitty, the anticipated phytoplankton were not produced. Bioassays showed that phytoplankton were limited by phosphorus and nitrogen. No evidence for an allelopathic effect was found. Macrophyte uptake was responsible for removing 36% of the nitrate added, and the sediment responsible for a part of the phosphate uptake. Some phytoplankton uptake was inferred from the increased zooplankton numbers in enclosures receiving phosphate and nitrate. Nutrient additions had no effect on macrophyte standing crop, as predicted, because the sediment provided an adequate nutrient supply. With decay of macrophytes and nutrient release, phytoplahkton increased in certain enclosures, but not others, probably as a result of large increases in zooplankton numbers and hence grazing pressure. The filamentous alga Rhizoclonium became abundant at the end of the season in enclosures receiving phosphate and nitrate, but did not appear to harm the macrophytes. Epiphytes were only visibly obvious in one enclosure. Failure to produce dense phytoplankton crops in the field led to a laboratory study of the effects of phytoplankton-induced carbon competition on macrophytes. Phytoplankton species were shown to have a smaller total resistance to CO2 fixation than macrophytes and hence greater photosynthetic rates under most CO2 concentrations. The boundary layer was the largest component of the total resistance in macrophytes, suggesting that the thin leaves of many macrophytes were a response to this rather than an aid to diffusion. The linear leaves of other species could be adaptations to reduce the boundary layer thickness. A pH-drift technique confirmed that the best phytoplankton species were more efficient at carbon removal than any macrophyte shoots. The macrophytes were even less efficient when the whole plant was considered. The carbon compensation point was shown to rise under the low light conditions that would be found under a dense phytoplankton crop. Macrophytes showed seasonal changes in carbon extractive ability, but the range was less than published data for phytoplankton from a lake, probably because the latter consists of a series of populations, which are closely adapted to the prevailing conditions. Different leaf types of heterophyllous macrophytes had different CO2 compensation points and one leaf type could use HCO3. A growth experiment confirmed that carbon competition with phytoplankton could have a detrimental effect on macrophytes.