Seasonality of macroalgae and epilithic diatoms in spring-fed streams in Texas, USA

A seasonal study of two spring-fed stream systems in south-central Texas wa s undertaken over a 15-month period from June 1996 to September 1997. Relat ive abundance of the epilithic diatom flora, percent cover of macroalgae, a nd several physical and chemical conditions were monitored in one 20-m stre tch in each of the streams at approximately two-month intervals. Six additi onal spring-fed stream segments were sampled in May 1996 for macroalgae and measured for the same conditions, in order to determine how representative the macroalgal floras of the study sites were of south-central and western Texas. Conditions in the two stream segments were relatively constant over the 15 months, except for maximum depth; this finding is similar those rep orted for other spring-fed systems worldwide. Sixty-eight diatom taxa and n ine macroalgae taxa were identified from the San Marcos River, which compar es to 46 diatom taxa and six macroalgae taxa from the Comal River. Several diatom and macroalgae species were significantly correlated to stream condi tions (e.g. Dichotomosiphon tuberosus and Hildenbrandia angolensis with day length, and Amphora pediculus with maximum depth). Twenty-one macroalgal ta xa were identified from the six additional spring-fed steam segments. Condi tions in these streams were very similar to those measured in the San Marco s River and Comal River, except that they had a higher average pH ((x) over bar x = 8.1 versus (x) over bar x = 7.6). The epilithic diatom flora of th e San Marcos River and the Comal River contains many of the diatom and some macroalgal taxa commonly reported from springs globally.