Abstract

The long-term impacts of declining water quality from coastal development on macro-algal communities can be devastating, but are rarely known because of lack of baseline studies. This study examines the effect of increased sediment and reduced water quality over 35 years in an Australian temperate coastal embayment. The algal assemblage on Crawfish Rock in northern Western Port was surveyed in 1967–1971 and in 2002–2006. During the 1980s, water quality declined following large-scale seagrass loss. In 1971, the Rock had a rich algal flora with 138 recorded species, including 97 species of Rhodophyta. The biomass and cover of canopy and understorey species were measured at sites of strong and slight current on a depth gradient. In 1971, fucoid or laminarian canopy species were dominant from ∼1–8 m depth, and an algal understorey extended from the intertidal zone to 12–13 m depth. In 2002–2006 the canopy species extended to only 3 m depth and the algal understorey to ∼4 m depth, and 66% of the algal species had disappeared, although a few additional species were present. Persistent, sediment-tolerant species included several phaeophycean canopy species, some chlorophytes (Caulerpa spp.) and a few rhodophytes.