#72 - Nov 2023 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz SEAWEEDSNov/Dec: Late spring, early summer. Warming, drying. Dead seaweed off the beach makes good mulch. Weeds? Weeds are plants in the wrong place. Some seaweeds are weeds. Our native seaweeds are in danger from exotics brought from afar by ships. Google Caulerpa NZ Macroalgae: Seaweeds are not weeds but sea plants (macroalgae) that live somewhere along a line from the ‘spray zone’ (just above high tide, never immersed, contacted by saltwater spray and freshwater rain), through the ‘intertidal zone’ (saltwater emersed twice daily, sun-baked, contacted by freshwater rain) into the ‘subtidal zone’ (below low tide, always saltwater immersed). Each of the thousands of seaweed species is adapted to life in a narrow range of these contrasting conditions. Travel the short distance along this line from land to sea and the mix of seaweed species changes continually and abruptly. Holdfast: So it stays in the right spot on this line, a seaweed anchors itself to a rock with its holdfast (anchor, not root). Held there it thrives, detached it dies. So, many seaweeds on a rocky beach, none except the dead/dying on a sand/gravel beach. Thallus/bladders: A seaweed’s body (thallus) is tough and flexible, so it bends with the waves. Some have floats so they ‘hang’ upwards, reaching for the light. Photosynthesis: Like land plants, seaweeds make their own food by photosynthesis – they capture light energy to make sugars. Seaweeds come in one of three colours - green, red or brown. Chlorophyceae: The green seaweeds contain chlorophyll – this captures blue light (450 nm) and the longer wavelength red light (665 nm) that doesn’t penetrate far in water. So, greens are mostly shallow. Rhodophyceae: The red seaweeds contain phycocyanin and phycoerythrin – these capture the yellow/red light (610–620 nm) and the shorter wavelength green/yellow light (530-570 nm) that penetrates deeper. So, reds are mostly intertidal. Phaeophyceae: The brown seaweeds contain fucoxanthin which captures the shorter wavelength blue/green light (450-540 nm) that penetrates deeper still. So, browns are mostly down into the subtidal. Ecology: Seaweeds are food for a myriad species including kina. Subtidal kelp forests offer safe refuge for many young fish. If an area is over-fished, the kina’s predators go, the kina proliferate and chomp all the kelp, so there’s no safe place for young fish. Wellington harbour is badly overfished, so its kelp forests are disappearing, so no safe place for young fish, so fewer fish...___________________________________