The role of photosynthesis/light relationships in determining lower depth limits of Characeae in South Island, New Zealand lakes

1. The maximum depth of colonization of aquatic macrophytes (Zc) was investigated in eighteen South Island, New Zealand lakes. The downward attenuation coefficient for photosynthetically active radiation (Kd(PAR)) was calculated and the spectral characteristics of the lakes determined with a spectroradiometer. 2. Characean algae dominated the deepest communities in sixteen of the study lakes. 3. Zc was significantly related to Kd(PAR) by the relationship Zc = 4.5/Kd– 2.2. 4. From measurements of the photosynthetic properties of Chara corallina (Kl. ex Willd., em R.D.W.) and incident radiation over the course of a year we calculated the depth at which daily net photosynthesis would be equal to zero for each day of the year. An annual average of this depth was significantly related to Zc with anr2 of 0.86. 5. Correcting Kd(PAR) for spectral quality and taking into account the potential absorption spectrum of a characean meadow did not improve the relationships. 6. We suggest that relationships established between Kd(PAR) and Zc of characean algae in South Island, New Zealand lakes can be explained to a great extent by light limitation of photosynthesis.