ENDOMYCORRHIZAE OF ISOETIDS ALONG A BIOGEOCHEMICAL GRADIENT

Endomycorrhizae of aquatic plants may be important in phosphorus uptak e and carbon exchange in lakes, but the environmental controls on myco rrhizal distribution are not known. We examined biogeochemical variabl es that were correlated with aquatic endomycorrhizae of isoetid-type m acrophytes in an oligotrophic, softwater lake. Endomycorrhizal infecti on was greatest in the shallow stations with high sediment redox poten tial and lowest in the deeper stations where there was low redox poten tial and high sediment organic content and porewater P levels. There w as a significant (r = +0.93, P < 0.05) positive correlation between pe rcentage of hyphal infection of the roots and a root ergosterol (a spe cific fungal sterol) index determined per root mass. Fungal vesicle in fection was also positively correlated with the root ergosterol index, although not significantly (r = +0.76, P = 0.14). Furthermore, the ro ot ergosterol index was significantly correlated with plant rosette de nsity (r = +0.97, P < 0.05). An increase in rosette density of isoetid s increases the number of lacunae transporting oxygen to the roots, pe rhaps increasing fungal infection. The root ergosterol index was also significantly (P < 0.05) correlated with sediment redox status, porewa ter phosphate, solid phase iron-bound P, exchangeable inorganic P, and inorganic adsorbed P. There were no significant correlations between above- or belowground biomass or porewater NH4+ with the root ergoster ol index, and porewater DOC was weakly correlated (P = 0.08) with the ergosterol index. Isoetid-type plants are common in oligotrophic softw ater lakes, and knowledge of environmental variables that are associat ed with endomycorrhizae will facilitate in management and restoration of these types of submersed vegetation.