Backgrounder

Electron microscope photo of the dinoflagellate, Karenia brevisulcata, responsible for mass marine life mortality and contributing to human respiratory syndrome during the 1998 toxic episodes in Wellington Harbour.

Major harmful algal blooms (HAB) recorded from 1950 to 2002: S, nuisance “slime” events; A–I, other major harmful algal events. IPO phases are marked. (See The Climate Update, May 2000 and October 2000, for discussion of the IPO.)

Variation of SOI and northwesterly wind on northeast coast of North Island in summer seasons, 1961–2000.

Occurrence of major harmful algal blooms in New Zealand: is there a link with climate variations?

Hoe Chang and Brett Mullan, NIWA, Greta Point, Wellington

Marine algae grow rapidly under conditions of high light intensity, plentiful nutrients, and favourable water temperatures. When microalgae build up to massive numbers they are said to form blooms (collectively termed ‘red-tides’ in the past). In winter, although nutrients are plentiful, the deep mixing caused by winds and turbulence generally inhibits algal growth. As the ocean mixed-layer shoals in spring, microalgae close to the surface waters become exposed to higher light intensity and reasonably high nutrients. This provides the conditions for rapid growth, and typically diatoms are the first to take advantage of this change and form strong blooms in coastal waters around New Zealand. In summer, as the surface waters become warmer, and thus more stable, nutrients become scarce, and so algal blooms are generally not expected.

However, in the last five decades a relatively large number of irregular, but widespread, harmful algal blooms (HAB) were recorded in summer. Many of these blooms, dominated by dinoflagellates (left), were very noticeable, particularly to the general public, through their effects such as visible discoloration of the water, foam/slime production, fish or marine fauna kills, or poisoning to humans through seafood consumption.

It is now clear that almost all major HAB recorded in the last 50 years coincided with El Niño Southern Oscillation (ENSO) events. Between 1950 and 1977, four nuisance “slime” events were reported in New Zealand, mainly during La Niña events in the negative phase of the Interdecadal Pacific Oscillation (IPO). From 1978 to 2002, twelve major HAB outbreaks were reported, most on the North Island northeast coast during El Niño events in the positive phase of IPO (see graph below). It is thus possible that the IPO is modulating algal bloom activity around New Zealand.

A relatively high proportion of these blooms, particularly those recorded during the El Niño events from 1978 to 2002, caused widespread fish and marine fauna kills. On some occasions these toxic episodes also posed human health risks – either direct food poisoning through shellfish consumption or human respiratory syndrome through exposure to toxic ‘aerosol’ generated by the blooms. The question arises from observations of these mostly summer HAB events during ENSO: what is driving these “unseasonal” and very widespread major blooms in summer in New Zealand?

Wind records collected in New Zealand over many years clearly show the strengthening during El Niños of northwesterlies on the North Island northeast coast (above), and of southwesterlies on the South Island northwest coast. Along-shore winds produce a transport of water to the left of the wind (in the southern hemisphere), and therefore offshore for these two coastline segments. Strengthening of local winds during El Niño in summer thus leads to an increase of upwelling intensity and surface nutrient enrichment in these regions. The cold, nutrient-rich waters brought up from the deep in these areas in summer, in particular off the North Island northeast coast when nitrate nutrient is normally undetectable, have been suggested to promote growth and eventually lead to the build-up of the very widespread, strong HAB events.

Thus, wind changes during El Niño summers can generate a sustained nutrient supply during a season that is normally nutrient poor, and encourage the development of harmful algal blooms around New Zealand.

Stunning red tides can be seen clearly in this aerial photo taken near Langs Beach on the east coast between Auckland and Whangarei on 1 December 2002. (Photo: Miriam Godfrey)