Temperature and nutrients are fundamental, highly nonlinear drivers of biological processes, but we know little about how they interact to influence growth. This has hampered attempts to model population growth and competition in dynamic environments, which is critical in forecasting species distributions, as well as the diversity and productivity of communities. To address this, we propose a model of population growth that includes a new formulation of the temperature-nutrient interaction and test a novel prediction: that a species' optimum temperature for growth, T , is a saturating function of nutrient concentration. We find strong support for this prediction in experiments with a marine diatom, Thalassiosira pseudonana: T decreases by 3-6 °C at low nitrogen and phosphorus concentrations. This interaction implies that species are more vulnerable to hot, low-nutrient conditions than previous models accounted for. Consequently the interaction dramatically alters species' range limits in the ocean, projected based on current temperature and nitrate levels as well as those forecast for the future. Ranges are smaller not only than projections based on the individual variables, but also than those using a simpler model of temperature-nutrient interactions. Nutrient deprivation is therefore likely to exacerbate environmental warming's effects on communities.

The forage f i s h assemblage o f the Brazos River was sampled a t one l o c a t i o n above and four locations below Possum Kingdom Reservoir each month f o r one year. A t o t a l o f 13,936 i n d i v i d u a l s representing 18 species, 14 genera and f i v e families were collected. Five o f these species, N o h o p h b h m i n e u . 6 , N o m p h a t h e h i n o k f a , H y b o g d h u s p h c i b , Hybopbh a a . t L v a L A and Cyphinodon m b m 6 & v W were found only upstream from Possum Kingdom Reservoir. H y b o g n a b phcitud accounted f o r 45% o f f i s h captured a t the upstream s i t e . I n contrast, nine species were c o l l e c t e d exclusively i n the tailwaters, including Etheohhma h p e o t a b d e , P e h c i m h c i e m , Campohtoma ano&, N o h o p d venubtub, P h e p h d e s notaak.6, L a b i d e s t h a b i~~d~6 and F u n m no-. Only f o u r species were obtained a t a l l f i v e stations--& b u s h a 6 6 i n i s , M e n i d i a b e h y U n a , P h e p h d e s vLg&x and No&oph &,tkenbh. N. -!~i9~emh was the most c o m n t a i l w a t e r species equal 1 i n g 87.9% o f the t o t a l downstream catch. Median species d i v e r s i t y indices ranged from 0.129 t o 0.451 f o r the f i v e stations, w i t h the highest value a t the upstream station. Coefficients o f b i o t i c s i m i l a r i t y ranged from 0.20 t o 0.62, w i t h the upstream s t a t i o n c l e a r l y being the most d i s s i m i l a r . These findings are d i scussed r e l a t i v e t o environmental e f f e c t s of hypol imnetic release o f reservoir waters.

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