Aquatic Botany 70 (2001) 9–27 Photosynthetic characteristics of sub-tidal benthic microalgal populations from a temperate, shallow water marine ecosystem Brett R. Light ∗ , John Beardall Department of Biological Sciences, Monash University, Wellington Road, Clayton, 3168 Vic., Australia Received 26 November 1999; received in revised form 4 July 2000; accepted 13 November 2000 Abstract The photosynthetic characteristics (α B , P B max and I k ) of sub-tidal benthic microalgal populations from Port Phillip Bay, southern Australia, have been examined using an oxygen exchange procedure from intact sediment cores. Photosynthesis–irradiance (P–I) parameters were measured at two water depths and three sites over the course of an annual cycle, enabling strategies for spatial and temporal (seasonal) photoacclimation to be determined. The respective roles of photosynthetic photon flux density (PPFD) and water temperature in regulating P–I parameters are also discussed. The benthic microalgal population exhibited marked acclimation to in situ PPFD, which varied as a function of seasonality, water depth and site. Examples of benthic microalgal photoacclimation strategies based on an increase in photosynthetic unit size are evident in Port Phillip Bay. P–I parameters followed a unimodal seasonal pattern with maximum values generally occurring in the late spring to early autumn period and minimum values generally occurring throughout winter months. A strong relationship exists between P B max and α B , which is independent of PPFD and temperature. Variation in benthic microalgal P B max was determined principally by variation in PPFD whereas water temperature exerted the greatest influence on α B . Both parameters combine in the form of the parameter I k which provides an accurate photoacclimation index in Port Phillip Bay. Compared to deep water sites, shallow water sites gave poor spatial and temporal signals, indicating the importance of environmental factors other than PPFD in regulating P–I parameters in these physically constrained habitats. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Benthic microalgae; Photosynthesis; P–I parameters; Port Phillip Bay; Sub-tidal ∗ Corresponding author. Present address: Environment Protection Authority, 27 Francis St., Melbourne, 3000 Vic., Australia. Tel.: +61-3-9616-2315; fax: +61-3-9614-3575. E-mail address: brett.light@epa.vic.gov.au (B.R. Light). 0304-3770/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0304-3770(00)00146-7