What's all that green stuff on the beach? by John Ogden

Environmental News Issue 15 Winter 2008

After several storms this winter masses of green ‘mossy’ seaweed have washed up on the beaches at Tryphena. According to local residents this is a new phenomenon — in the past storms washed up sea-grass and brown seaweeds, not this stuff. Some residents have carted away loads for use as a mulch on their gardens, but Karen Walker collected a smaller sample in a zip-lock bag ! The specimen was sent to Ewen Cameron and Mike Wilcox at the Auckland Herbarium, and has now been identified as Microdictyon mutabile. It is composed of many minute green cells forming ‘twigs’ arranged in loose sheets, but cross-connected by other cells, so that the whole thing is a spongy mass. It is very pretty under the microscope!
Why this plant has suddenly become so common, and what effect it is having on the marine ecosystem is unknown at present. Many green seaweeds like fresher water than brown or red seaweeds, and some are stimulated by higher nutrient levels (eutrophication). Where higher nutrients might be coming from at Shoal Bay is unknown, but if they are linked to the high coliform counts sometimes recorded there, then they might relate to septic tanks, high water-tables and run-off after heavy rain. On the other hand ‘natural’ marine processes could be at work — without more data we can only speculate. However, environmental science has taught us that the natural system is often quick to respond to environmental changes, which we find difficult to detect by relatively infrequent monitoring with instruments. Short-lived, fast-growing organisms such as algae often provide sensitive indicators of environmental change.