The phenology of Ruppia cirrhosa (Petagna) Grande and Chara sp. in a small temporarily open/closed estuary, South Africa

The study investigated the phenology of Ruppia cirrhosa (Petagna) Grande and Chara sp. in the East Kleinemonde Estuary, a warm temperate temporarily open/closed estuary (TOCE) along the south-eastern coastline of South Africa. Monthly growth and sexual reproduction were monitored over a 17 month period, including environmental conditions. The estuary was closed to the sea for the duration of the study and inundation of the submerged habitat ranged from 6 to 86 cm. Both species began to germinate in Autumn 2009 (April and May) once the submerged habitat had been flooded for one month. R. cirrhosa began to produce viable seed after five months and production occurred over a four month Spring-Summer period. Maximum seed production occurred in late Spring (26,242 ± 3401 seeds m−2). It took C. sp. three months to start producing viable oogonia with production taking place over seven months, from late winter late Summer with a maximum of 196,998 ± 48,004 oogonia m−2 produced. Peak biomass was attained during Spring to mid-Summer when R. cirrhosa reached 2248 ± 388 g DW m−2 and C. sp. 142 ± 34 g DW m−2, eight and six months after germination, respectively. Multivariate analysis showed that both biomass and reproductive output increased as water level and pH did. In conclusion, the high biomass and reproductive output of both submerged species under continued water permanence maintains large seed banks for habitat persistence during open mouth states when submerged beds are lost through exposure. This is an important survival strategy in small estuaries where water level fluctuates in response to unpredictable mouth breaching and mouth closure events.

► The growth and phenology of two submerged macrophytes were monitored in a small estuary. ► The estuary has large variations occur in water level and salinity. ► Water level increased life cycle length and reproductive output. ► Plants make use of stable conditions in an unpredictable environment.