The chemical and isotopic data from sea ice collected over a wide area of the Weddell Sea, Antarctica, during the austral summer/early autumn illustrate the range of environmental conditions under which ice algae grow. A range of ice types and features were sampled including intact and layered ice floes and surface ponds. Sea ice communities were found in all these environments but the highest biomasses were found either at the base of ice floes, or in the interior of layered floes with quasi-continuous horizontal gaps at or shortly below the water level. In the layered floes, particulate organic carbon (POC) measured in the ice layer immediately overlying the gap water (280 to 6014 μmol dm-3) was in excess of what would be predicted if algal growth had occurred in a closed environment. The chemical composition of the gap water was strongly affected by biological activity in the overlying ice, which acts as a physical support for the algae retained within its matrix. The lowest range of POC (27 to 739 μmol dm-3) conformed to predictions of algal growth in a closed system and samples were collected from the interior of ice floes where there was essentially no potential for nutrient exchange. The surface ponds displayed nitrate (NO3-) exhaustion and total dissolved inorganic carbon (∑CO2) reductions consistent with nutrient limited algal growth. The stable carbon isotopic composition of the particulate organic matter (POM) across all habitat types sampled (δ13CPOC -10.0 to -27.3‰) displayed a wide range but was much less variable than the range of POC concentrations might have implied. The assumption that the highest biomass of algae in sea ice will result in the most positive δ13CPOC values cannot be generally applied. The isotopic composition of dissolved inorganic carbon (δ13C∑CO2) in gap waters and surface ponds varied from 0.15 to 3.0‰ and was shown to be commensurate with the changes predicted from NO3- deficits caused by algal growth.