Photorespirometry has long been used to examine primary production of aquatic microand macroalgae. Despite a growing number of studies examining in situ primary production in soft sediment ecosystems, little has been done to test in situ primary production on rocky reefs. This disparity may be due, in part, to a lack of a suitable photorespirometry apparatus able to be sealed to rocky reef surfaces. To compare laboratory and field-based primary production we designed and tested a

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... er. Because the device fits securely to rocky reefs, we were able to test natural assemblages and monospecific stands during immersion in situ. These were then compared to similar algal assemblages in laboratory conditions over irradiance levels from 100 to 2000 µmol m -2 s -1 . Primary productivity of both canopy-forming fucoid algae and understory corallines was greater in situ than under lab conditions, particularly at irradiance levels above 1500 µmol m -2 s -1 . Comparisons between laboratory and in situ conditions suggest fundamental differences in dynamics of productivity and point to a vital role of light delivery. Furthermore, the increasing productivity down a shore-height gradient shows that a combination of greater biomass and greater numbers of macroalgal species significantly increases primary productivity of these macroalgal assemblages. KEY WORDS: Primary productivity · Macroalgae · In situ · Irradiance · Rocky intertidal · Fucoids Resale or republication not permitted without written consent of the publisher Editorial responsibility: Antony Underwood,