Gone West; first observations of surf-zone diatom Anaulus australis accumulations on the West Coast

Abstract

Surf-zone diatom Anaulus australis accumulations, recognised by brown discoloured water, are typical of long dissipative beaches on South Africa’s south and east coasts, notably those with a nutrient supply from adjacent dune-field aquifers. Anaulus spend the daytime in the water column but are nocturnally epipsammic. Previous surf-zone diatom surveys of the West Coast (Cape Point to Alexander Bay to the Skeleton Coast) recorded benthic A. australis but none in the water column, nor any visible discoloured water accumulations, thus concluding a different mode of behaviour in it being epipsammic throughout the day (Bate & Campbell 1990). Sixteen Mile Beach MPA falls within the West Coast National Park, on walking it in January 2022, we discovered that “things” may have changed. We observed 17 brown discoloured patches, about one per 1.5km. Water samples were collected in a washed-up plastic bottle and the diatoms microscopically verified as A.australis. The next question is, how long has it been there? Sentinel satellite images at time of sampling clearly showed the patches as did others taken over the past five years. In turn, satellite images of West Coast sites where epipsammic Anauluscells but no visible water column accumulations were previously recorded (Bate & Campbell 1990), indicated patches occurring at least 10 sites from Melkbos to the Skeleton Coast over the same time period. The recipe for Anaulus patches seems similar to that on the south and east coasts but weather conditions before and during their formation suggest subtle differences in accumulation dynamics. Lastly, if the accumulations are a new occurrence, what are the implications and benefits to surf-zone fish and invertebrates in the MPA?