Mesozooplankton community responses to a large-scale harmful algal bloom induced by the non-indigenous dinoflagellate Lingulodinium polyedra

Our understanding of how zooplankton community composition varies in relation to harmful algal blooms remains limited, particularly in ecosystems where toxin-producing algae may have been introduced through anthropogenic activities. Harmful algal blooms (HABs) naturally occur on the coast of southern Africa, where they are predominantly associated with the cold Benguela region. In the warm-temperate waters east of Cape Agulhas, HABs occur rarely and red tides are mostly associated with the non-toxin producing dinoflagellate Noctiluca scintillans. Blooms of N. scintillans may cause water discolouration, but this is generally short-lived with limited impact on the ecosystem. However, in December 2013 the eastern Agulhas region experienced an extensive HAB, which persisted for ca. 4 months and affected >500 km of coastline, from Wilderness to East London. This unprecedented event was caused by the non-indigenous toxin-producing dinoflagellate, Lingulodinium polyedra. The impact on the coastal seas was widespread and severe, with instances of low dissolved O2 levels and fish kills being reported at the time in the broader Algoa Bay area. This study investigated the impact of the L. polyedra bloom on the mesozooplankton of Algoa Bay and reports the successive changes in zooplankton community composition and biomass observed from July 2013 to July 2014. The bloom impacted species diversity and richness, with a marked shift in dominance from a calanoid copepod dominated community to one dominated by microzooplankton (specifically cyclopoid copepods, tintinnids and cladocerans), over the period November 2013–March 2014. Calanoid copepod abundance was significantly reduced throughout Algoa Bay with the progression of the bloom, and this significantly impacted the total zooplankton biomass of the region. The results of the study suggest that harmful algal blooms have a negative impact on zooplankton communities, with notable implications for the higher trophic levels of the coastal pelagic ecosystem. [Display omitted] •During L. polyedra bloom, crustacean zooplankton was dominated by small Oithona spp.•The abundance of medium to large calanoid copepods declined during the bloom.•The tintinnids Favella sp. was only recorded during the L. polyedra bloom phase.•Zooplankton community composition before and after the L. polyedra bloom were similar.•L. polyedra is likely to bloom again in Algoa Bay.