Miriam Godfrey

Stunning pink and red algal blooms coloured surface waters in many parts of the Hauraki Gulf in late November and early December 2002, from Bream Bay to Auckland’s North Shore. As they drifted with the tides, the blooms also invaded beaches and bays, forming extensive slicks.

The first outbreak was reported near Waiheke Island in early October, when several algal species—including some harmful ones—bloomed simultaneously. Anyone curious enough to collect samples of the blooms would have noticed two things: the largest organisms, up to 2 mm across, resem­ble sago, and, if collected at night, the mere act of disturbing the seawater containing the bloom creates a scin­tillating display of tiny sparkles.

The sago-like organism is a dinoflagellate alga known as Noctiluca scintillans. It is found in warmer wa­ters throughout the world, and is well named, for its scientific name literally means sparkling night-light. Most dinoflagellates have photosynthetic pigments in their cells but Noctiluca is an exception. It lacks chlorophyll, and cannot therefore capture the sun’s energy for photosynthesis. Instead, it feeds on other living cells, and when they bloom, Noctiluca blooms. Blooms can reach several million cells per litre of seawater. In these numbers it colours the sea, creating the spectacular red hues seen from the air.

Noctiluca’s cells are peach-shaped, ranging mostly between 0.4 and 0.8 mm. Each has an oral pouch for feeding. At the edge of this pouch is a prominent striped tentacle. The pink colour is caused by minute globules of carotenoid pigment in the cytoplasm.

Bioluminescence is caused by a natural enzyme reaction to agitation. Large areas of surface water glow in the dark when cells are disturbed either by wind or wave action.

Where blooms have accumulated on beaches, cells, even when semi-dry, can sparkle in the dark in the shape of footprints when trodden on.

The purpose of the biolumines­cence is unclear. Perhaps the sudden, strong, synchronous sparkling of an enormous number of cells triggered by approaching fish scares predators away.

Noctiluw is a voracious feeder, ingesting pretty much any food particles it can capture. Among the items which have been found in its diges­tive vacuoles are microalgae, protozoans, copepod eggs, small crustaceans, and even pine pollen.

In the Waiheke algal event, Noctiluca appeared in blooms of a group of toxic dinoflagellates which have normal photosynthetic pig­ments (Karenia mikimotoi, K. brevisulcata, and Gymnod­inium spp.). Large numbers of fish and a substantial number of paua were re­ported killed at Orewa and at an aquafarm at Kennedys Bay, Coromandel Peninsula.

Karenia mikimotoi and K. brevisulcata were thought to be the likely culprits, as both species have previously been associated with marine kills in New Zealand. Inter­estingly, microscopic exami­nation of Noctiluca collected from the Waiheke area during the bloom revealed very few toxic cells in its food vacuoles, even though cell concentrations of the toxic species were 1000 times greater than those of Noctiluca. It seems Noctiluca exercises some discrimina­tion over what it eats.

Around Bream Bay, in late November, a small number of cysts produced by Alexandrium cf. catenella, a dinoflagellate known to make paralytic shellfish poison (PSP), were detected in Noctiluca’s food vacuoles. The presence of these cysts suggests Alexandrium was being concentrated and its toxin passed on to organ­isms higher up the food chain, but since only a small number of cysts were found it is unlikely that they caused health problems in the area.

Noctiluca itself is not toxic to marine life, but it is possible that the build-up of ammonia in food vacuoles of Noctiluca cells has a negative impact on near-shore fish and aquaculture.

Other Noctiluca blooms have been spotted in recent months by New Zealand Custom Service aircraft conducting patrols around the coast. The first was seen in Hawke Bay on October 11, and extensive blooms were spotted off Kaikoura and Christchurch on Oc­tober 17. On November 26 several other major patches were visible in the outer Hauraki Gulf, and unusual coloured slicks (presumably caused by Noctiluca) were sighted by a fishing boat near the Chatham Rise.

Noctiluca blooms are annual events, usually happening in spring and early summer, but are not normally as profuse and as frequent as those of late 2002. Since Noctiluca thrives when its food supply is plen­tiful, other, less conspicuous microalgae have probably had a good year too.

The El Nino conditions that New Zealand has been experiencing may have played a part in promoting a higher than usual level of algal growth.