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Algal blooms wreak havoc in the Hauraki Gulf

On 11 October 2002 tens of thousands of fish were reported dead on Orewa beach north of Auckland. They included a whole range of shallow, midwater, and bottom-dwelling estuarine species (e.g., parore (mangrove fish), flounder, yellow-eyed mullet, eel, goby, and spotty). About 8500 paua at a marine farm at Kennedys Bay on the eastern Coromandel Peninsula were reported killed, even though seawater fed into the land-based aquaculture facility was pre-filtered to remove algal cells. What was causing the mass deaths of all these marine animals?

Water samples collected between 9 and 16 October from several areas around Orewa, Waiwera, north of Waiheke Island in the Hauraki Gulf, and also at Kennedys Bay outside the Gulf, showed the presence of several species of algae, Karenia and Gymnodinium. Karenia cell concentrations ranged from 1.8 to 10.7 million cells per litre around Orewa and Waiwera, up to 32.1 million cells per litre near Waiheke Island, and 6.7 million cells per litre in Kennedys Bay. While Karenia mikimotoi was the dominant species, there were also small numbers of Karenia brevisulcata, Gymnodinium pulchellum, and a possibly new Gymnodinium species.

Stunning "red tides" can be
seen clearly in this photo.

The build-up of high concentrations of Karenia and Gymnodinium spp. at the time of the outbreaks had negative effects on fish (e.g., damaging their gills and resulting in death), but cell concentrations alone do not explain the mass mortality of paua at a marine farm where intake seawater was pre-filtered. It is likely that toxic substances released from broken cells into the filtered seawater killed the paua. On 9 October a brown to tomato-soup-like surface discolouration was observed north of Waiheke Island, showing the presence of a non-photosynthetic dinoflagellate Noctiluca scintillans. At the height of the bloom the Noctiluca cells were surrounded by tens of millions of dominant Karenia spp., however, the cells examined were either completely free of food particles or only had traces of partly digested cells from potentially harmful species. A very small number of Noctiluca cells examined contained diatoms, copepod eggs, and crustaceans. It is highly likely that Noctiluca actively avoided feeding on these gymnodinioid species (K. mikimotoi, K. brevisulcata, Gymnodinium pulchellum, and Gymnodinium sp.) because of their toxicity to marine organisms.

A live specimen of
Gymnodinium sp.

Cells of Karenia mikimotoi

All the gymnodinioid species recorded in these blooms are naked dinoflagellates (i.e., their cells lack “armoured plates” or a proper cell wall) that share many morphological features and are often linked to the deaths of marine organisms. Cells of both Karenia species and the possibly undescribed Gymnodinium species are all dorsoventrally flattened, but cells of G. pulchellum are only slightly flattened. The cells of all four gymnodinioid species mentioned above are slightly smaller in width than in length (25–35 µm long, 22–33 µm wide), with K. brevisulcata the smallest (19–25 µm long, 18–22 µm wide). Both K. mikimotoi and K. brevisulcata have linear apical grooves (which are very short in the latter species), whereas G. pulchellum has a sigmoid-shaped apical groove. We do not yet know the precise shape of the apical groove in the undescribed Gymnodinium species.

Algal bloom in Leigh Harbour.

The position and shape of the nucleus in this group of gymnodinioid dinoflagellates are also important diagnostic features. Karenia mikimotoi tends to have either a vertically elongated or round nucleus in the left lobe of the hypocone (lower half of the cell), while K. brevisulcata has a horizontally elongated nucleus in the hypocone or (more rarely) a round nucleus in its left lobe. Gymnodinium pulchellum has a centrally located nucleus between the epicone (upper half of the cell) and the hypocone. Gymnodinium sp. stands out from all of these species because it has either a round or slightly elongated nucleus that occupies the epicone.

In the last decade or so several very widespread blooms have occurred in the Hauraki Gulf. Most of these blooms emerged in summer (e.g., the Cerataulina pelagica/Prymnesium calathiferum blooms of 1983 that reportedly caused fish kills, and the Karenia/Alexandrium blooms (K. mikimotoi, K. brevisulcata, Karenia sp., and Alexandrium minutum) associated with both neurotoxic shellfish poisoning (NSP) and paralytic shellfish poisoning). Even though the spring 2002 blooms were dominated by K. mikimotoi, including small numbers of other Karenia and Gymnodinium spp., no shellfish toxins have so far been detected.

During the summer blooms of 1992–93 a number of microflagellates (e.g., Chattonella marina, Heterosigma akashiwo, and Fibrocapsa japonica) known to produce a small amount of brevitoxins overseas were also recorded in the Hauraki Gulf. We are not yet sure whether this group contributed to the production of any NSP in shellfish collected in the region during the 1992–93 blooms.

However, we can be sure that the knowledge gained from identifying harmful and potentially toxic microalgae, and understanding their harmful effects on other marine life, is helping New Zealand's aquaculture industries. The negative effects shown by some of these organisms may also make them useful for culturing and isolating bioactive compounds that could be screened for future biotechnology endeavours.