We tested interactive effects of warming (+2°C) and browning on periphyton accrual and pigment composition when grown on a synthetic substrate (plastic strips) in the euphotic zone of sixteen experimental ponds. We found that increased DOC alone, or in combination with warming, resulted in a substantially enhanced biomass accrual of periphyton, illustrating that periphyton is capable of using nutrients associated with DOC, and by this may affect nutrient availability for phytoplankton. However, warming weakened the positive impact of DOC on periphyton accrual, possibly by thermal compensation inferred from altered pigment composition, and/or changes in community composition. Our results illustrate that impacts of climate change on algal growth are multiple, which could have implications for productivity and consumer resource use, especially in shallow areas in northern lakes.

Phytoplankton: Phytoplankton chla measurements by filtering 100 mL water, measured using spectrofluorometer (Perkin Elmer LS-55, MA, U.S.A.) with 433nm as excitation and 673nm as emission wavelength.

Periphyton: periphyton grown on plastic strips (60cm long, 10cm wide, and 0.75mm thick, excluded the algae growing on the top 10cm), measured pigments (chla, chlc, fucoxanthin) on three dates, at each date a strip that has been introduced is harvested, i.e. no strips that have been harvested before were measured. Duplicate or quadruplicate measurements per pond.