The Seaweed Saga Continues
by John Ogden

Back in winter 2008 (GBI Environmental News 15) I reported on the green ‘mossy’ seaweed being washed ashore in vast amounts at Tryphena. The problem has not gone away, and questions have been asked at the Auckland City Council and the last Community Board meeting – mainly along the lines of “what are we going to do about it?”
It seems to me that we need to know what is causing the problem, which has now been with us for nearly two years, before we can sensibly decide what to do. What we know seems to be as follows:
1. The seaweed is a ‘green macro-alga’. In fact more than one species is involved, but the mixture is mainly composed of Boodlea mutabile and B. umbilicatum. Both these are sometimes placed in the genus Microdictyon.
2. The problem commenced sometime in the 2007-08 summer.
3. It has not been recorded before at Tryphena “in living memory”.
4. It accumulates on the beach after SW winds and is very slow to decompose (at least 6 months).
5. It is not found at Okupu, or at Katherine Bay, or on any of the other beaches on Great Barrier Island that I have visited recently (on a special ‘seaweed’ expedition!).
6. So far as I have been able to find, similar ‘blooms’ involving this alga have not been recorded before in New Zealand.
7. However, a very similar phenomenon, involving the closely related alga Boodlea composita, has recently been reported from the North Wester Hawaiian Islands of Midway and Kure Atolls (Vroom et al. 2009).

Previously I suggested that, because many green algae in this family (Cladophorales) are highly responsive to fresh water, and to increased nutrients, that increased flooding and/or nutrient input from streams feeding into the Tryphena bays could have stimulated the growth of this alga.

Table 1. Median E. Coli counts in four streams entering Tryphena Bay, 2003 – 09. “Median’ means that 50% of the values exceed this level. Bold italics indicates median values exceeding the ‘Action’ level (550 E. coli/100ml), plain text indicates medians exceeding the ‘Alert’ (260) level, brackets indicate medians below the ‘Alert’ level. Periods dictated by available data, but covering the summer holidays.
Raw data from S. Tang,
Auckland City Council.

Data on E. coli counts from four streams entering sea in the Tryphena area (Table 1) show that all four streams had median counts above the ‘alert’ level, often well above (ie ‘action’ levels) for most of the recorded period. Between November 07 and April 08 all the streams, and especially Pah Beach and Mulberry Grove North streams, had very high coliform values. E. coli is a disease organism, and is not directly responsible for increased nutrient input, but if more E. coli are present, the chances are high that more nitrogen (and phosphate?) will also be present, since both are associated with faeces. I speculate that nutrients have been gradually increasing in the sea-water at Tryphena for several years, and suddenly in 2007-08 a threshold level was reached and Boodlea really took off. Once established it could take a while to disperse again, and may not do so if nutrient inputs are sufficient to maintain those in the sea.
The macroalgal bloom at Kure and Midway Atolls in the North-western Hawaiian Islands was in summer 2008 (Vroom et al. 2009). The native green alga Boodlea composita formed dense mats in the lagoons and decomposing masses on beaches. Photographs in the paper look exactly like Pah beach! Vroom et al. suggest that mild oceanographic conditions during winter 2008 may have weakened the lagoonal flushing systems, allowing elevated nutrient levels, and this, coupled with weeks of low wind and warm sea surface temperatures during the proceeding summer, fostered rapid growth of this normally non-blooming alga. This is a particularly timely comparison, because clearly human pollution was not involved in the Hawaiian case, but a combination of factors, including elevated nutrient levels, were implicated. This could also be the case at Tryphena – higher nutrient input coupled with reduced circulation in the bay due to unusual climatic conditions is a plausible cause, but will be difficult to ‘prove’.
Many residents have found that the seaweed doesn’t make good garden compost because it is very slow to break down. Mike Wilcox of the Auckland Museum puts a different twist on this. He writes: “ I am very interested in this Microdictyon mutabile situation you have. It seems you may have a potentially useful commercial resource on your hands as Microdictyon and other green algae of the order Cladophorales has been the subject of research into its good pulp and paper-making properties. The cells walls are of highly crystalline cellulose and some have chitin and silica. This could explain their resistance to decay and composting”.
All this may not answer the question “What are we going to do about it?”, but it does indicate the need for more monitoring of our streams and bays so that causes can be pinpointed in future.
Reference: Vroom, P. S. et al. 2009. Macroalgal (Boodlea compositae) bloom at Kure and Midway Atolls, Northwestern Hawaiian Islands. Botanica Marina 52: 361-363.