Persistence of submerged macrophytes in a drying world: Unravelling the timing and the environmental drivers to produce drought‐resistant propagules

Abstract

Under a warming scenario many permanent inland water bodies in the Mediterranean region have become temporary ones, so the persistence of submerged macrophytes depends on the timely production of drought‐resistant propagules. Phenological research measures the timing of ecological processes and allows the consequences of disturbances such as climate change to be evaluated. Among macrophytes, the charophytes are widely distributed benthic macroalgae and are particularly sensitive to environmental variations. The systems in which they live constitute valuable, threatened, and protected habitats (European Habitats Directive code 3140). This study established a methodological basis for determining which environmental drivers trigger the sexual reproductive phenology of charophytes. For the first time, circular statistics were applied to describe and compare seasonal patterns for submerged macrophytes. Over a period of 2 years, three charophyte species growing in two Mediterranean brackish ponds were monitored. Most of the sexual reproduction occurred during spring and summer, including the production of drought‐resistant propagules, before the harshest conditions arrived. Each species showed a distinctive phenological pattern, according to specific breeding systems, life cycles, and environmental tolerances and requirements. Deeper conditions and higher salinities delayed reproduction. Different rainfall intensities in autumn produced distinctions between years, as stronger rainfall reduced the salinity stress and stimulated the reproduction of plants. Interacting factors act as cues for charophyte reproduction and cause antagonistic effects on propagule production. Among them, the heat required for each phenophase peak is essential for understanding the phenology of a population and for predicting its long‐term persistence. Charophytes appear to be good candidates for tracking climate change in shallow ecosystems. Further phenological studies should consider more species, populations, and long‐term observations in order to predict climate‐change effects on water bodies, and to develop effective plans for management and conservation. Circular statistics is a potential tool for analysing phenology in the context of global warming, as well as for many other conservation issues.

Reproductive phenology of submerged macrophytes: A tracker of year‐to‐year environmental variations

“…Within the Albufera de València Natural Park, on the east coast of Spain, several shallow interdunal ponds were created in the 2000s as part of the restoration of the seashore dune front and its associated humid dune slacks (Armengol, Antón‐Pardo, Ortells, & Olmo, ). The Llacuna Nova del Canyar (Figure a), created in 2007 as a permanent pond because its excavation reached the groundwater table, presents a water level that mainly depends on rainfall (Calero et al., ). Its waters are oligohaline owing to the large influence of the sea; the pond is located only 175 m from the seashore and its salinity increased significantly throughout the study period (from 1.7 g/L in 2014 to 2.5 g/L in 2016, Rodrigo & Calero, ).…”

“…Thus, the hydro‐morphology and community composition of this created pond were equivalent and can be considered representative for this kind of Mediterranean permanent ponds. During the study period (2014–2016), the Llacuna Nova del Canyar held a medium diversity of submerged macrophytes growing in scattered meadows (Calero et al., , ; Rodrigo & Calero, ): five charophytes and two angiosperms (Figure d). Dense beds of Chara hispida (Ch hereafter) dominated the pond and coexisted with Chara aspera (Ca), Nitella hyalina (Nh), Stuckenia pectinata (Sp) , and the free‐floating plant Utricularia australis (Ua).…”

“…Fresh material of all species was carried in plastic bags inside a portable cooler to the laboratory. Plants were gently washed with tap water and selected (between 846 and 1395 individual plants per species in total) for observation under a binocular microscope (Olympus SZ61, 67.5 × maximum magnification) to detect sexual reproduction: flowers and fruits in angiosperms (Rodrigo & Calero, ), and ripe gametangia and ripe oospores in charophytes (Calero et al., ).…”

“…The distribution of the numbers of flowering and of fruiting species throughout each annual cycle were tested by using circular statistics to find out whether they were seasonal (Morellato, Alberti, & Hudson, ). These statistics constitute a powerful tool to test hypotheses in annual data and a developing field in the analysis of plant phenology both at the population and the community level (Calero et al., ; Le Stradic et al., ; Morellato et al., ). Instead of using a linear scale, this methodology represents the year as a circle, where time units are converted to angles (Zar, ).…”

“…Data were grouped into monthly intervals by dividing the circle into 12 sectors of 30° each. For detailed information about the application of these statistics in the phenology of submerged macrophytes, see Calero et al., .…”