Reconstructing historical marine populations reveals major decline of a kelp forest ecosystem in Australia

Managing changing ecosystems requires an understanding both of how the system is currently performing and of how current performance relates to long-term, often variable, natural dynamics. However, making such assessments usually relies on having long-term ecological datasets, leaving managers often reliant on assumptions because such information is relatively rare. This is particularly an issue in kelp forests, as these ecosystems are difficult to survey, and many monitoring programs are relatively recent (last 10–20 years). A historical time series was constructed of kelp (Ecklonia radiata) and sea urchin (Heliocidaris erythrogramma) populations using various field-collected datasets, beginning in the 1960s, for the large shallow embayment of Port Phillip Bay, Australia. Additionally, aerial photographs were sourced to calculate the extent of these algal beds, going back to the 1930s. Both surveys and aerial images confirm that between the 1930s and the 1980s, kelp once occurred as dense (21–58% cover) beds over large sections of reef at all surveyed sites. However, by the early 2000s, kelp cover had declined by between 59 and 98%. From 2005 to 2012, sea urchins became 250–420% more abundant and were observed to be directly consuming large areas of macroalgae, creating the so-called sea urchin barrens. Analysis of reef algal cover between the 1930s and 2014 indicates that increases in temperature and declines in rainfall—which, in Port Phillip Bay, influences salinity, nutrient inputs and algal productivity—are correlated with the declines in kelp abundance. While it is difficult to tease apart all possible drivers, the years 1997–2009 coincided with a 1 °C increase in average air temperature and the longest drought period (137 mm less rainfall than the annual long-term average) in this region in recent record.