Hawksbury Lagoon update Dec 2020

I’m sure many of you have noticed the green fringe around the edge of the lagoon. This is known as filamentous algae. These filaments intertwine forming a mat that resembles wet wool. Filamentous algae start growing along the bottom in shallow water or attached to structures in the water (like rocks). Oftentimes the algae will become detached from what it was growing on and float along the surface of the water like a rug. Once the algae is established it creates its own self-supporting mechanism through continual seasonal blooms and die offs. Each die off in the winter becomes the blanket of nutrient that supports extra growth for the next season. The sediment accumulation on the base of the lake, caused by these seasonal die offs becomes more and more intense with each season that passes. As bacteria break down this organic matter, they use up oxygen, leading to anoxic sediments (without oxygen) and produce the smelly hydrogen-sulphide gas. Excessive algal blooms can deplete the water of oxygen to such an extent that any fish present will die.

Unfortunately, once filamentous algae blooms occur, they can be difficult to control. Excess nitrogen and phosphorus in runoff from the catchment often drive these problematic blooms. This is why we endeavour to maintain the water level, and manage flushing of the lagoon thereby reducing the cumulative effects of increased algal blooms and hopefully increasing the overall health of the lagoon.

We have now confirmed our plans for an “open day” over the summer. It will be held on Sunday January 17th from 10am until approx. noon at the Inverary Reserve. We will have a continuous morning tea (at a small cost), a wide range of plants/shrubs for sale, along with photo cards and our famous tea towels. Activities will include a scavenger hunt for children and a few other surprises.

There will be a range of speakers including Shirley McKewen Chairperson topic: wetlands of NZ, Greg Kerr Maori Reserves Trustee Rep topic: Hawksbury lagoon history, Matt Dale Senior Environmental Advisor – Freshwater Management Te Rūnanga o Ngāi Tahu topic: water monitoring of Hawksbury Lagoon and Derek Onley Ornithologist topic: Hawksbury Lagoon birdlife. These presentations may incorporate a walk along the causeways. A donation towards our work would be much appreciated.

We hope to see many of you there. If the weather is awful it will be postponed to the following Sunday.

Shirley McKewen

Chair