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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. |
