Inter-annual stability of populations of annual aquatic macrophytes in temporary freshwater rockpools of the Maltese Islands in relation to environmental stochasticity

Abstract

The ecology and hydrology of small cupular pools in the Mediterranean region, and in regions with a Mediterranean climate, have received comparatively little attention as most studies concerning Mediterranean temporary ponds (MTP) have focused on larger wetlands. The present study covered a seven-year period and aimed to fill a number of knowledge gaps concerning the dependence of rockpool hydrology on morphometric and meteorological factors and concerning the interaction of hydroperiod indicators (number of hydroperiods each year, duration of longest hydroperiod and hydroperiod index) with germination, growth and interannual population fluctuations of pool macrophytes. Hydrological, morphometric and meteorological data collected during the first six years of the study (wet seasons of 2001/2002 to 2006/2007) were used to construct simple models predicting the values of hydroperiod indicators. The models were validated using data from the 2007 /2008 wet season, predicting hydro period indicators that were not significantly different from the ones actually recorded during that wet-season. The explanatory power of the three models ranged from 70.5% (number of hydroperiods) to 85.9% (Hydroperiod Index) suggesting that a proportion of the variability in hydroperiod indicators is attributable to factors that had not been included in the models. The models also provided an indication of the relative importance of the factors considered, with total rainfall during November-December and amount of solar radiation in winter representing two significant predictors of hydroperiod indicators. Populations of most pool macrophytes were characterised by a broadly unimodal population growth with comparatively rapid growth and decline phases. The macrophyte communities in different pools were related to the hydroperiod index as pools with longer hydroperiods supported submerged communities of Zannichellia melitensis-Callitriche truncata-Chara vulgaris whilst pools with shorter, more fragmented hydroperiods were dominated by an amphibious assemblage dominated by Elatine gussonei and Ranunculus saniculifolius. Very shallow pools with very short hydroperiods were colonised by Triglochin laxiflorum and Juncus hybridus and a variable assemblage of opportunistic annual species. The propagule banks of the pools followed in this study were characterised by a strongly asymmetrical vertical distribution of seeds and oospores, with 63% to 96% of propagules, across all pools and sampling periods, being situated in the top 2cm of sediment. The high correspondence between the species composition of the seed bank and that of the established vegetation indicated strong interdependence between the two. Significant depletion of the seed bank of all pools was noted during the dry season and was partly attributable to attrition by wind and removal by predation. Specific germination and growth studies on Elatine gussonei, a representative amphibious species, indicated relative comparability of growth patterns, biomass accumulation and reproductive effort (RE) in 'Early flooding' (September) and 'Late flooding' (November) treatments. Conversely, plants from pots subjected to 'February flooding' treatment produced significantly fewer flowers, fruits and seeds, and less biomass. Increasing dry weight of plants was correlated with increasing seed production. Seeds buried under a layer of sediment germinated later than seeds placed on the surface and no germination was recorded from seeds buried under more than 2cm of sediment. The effect of projected climatic change on rockpool hydrology was investigated through the construction of three future outlooks ('Most optimistic', 'Optimistic' and 'Pessimistic') based on IPCC data for Southern Europe and the Mediterranean. The number of hydroperiods in all three projected outlooks was significantly higher than that recorded during 2007 /2008 (P <0.05) whilst the duration of the longest hydroperiod was projected to be significantly shorter in a warmer and drier climate (P <0.05). In general, Elatine gussonei and species with similar ecological strategies will be expected to persist in the rockpools of the Maltese Islands under each of the three future outlooks of climate. In the case of the 'Most optimistic' outlook, a population that was reduced to 16% of the carrying capacity by a possible intra-annual drought or other disturbance would potentially recover fully within three years. Conversely, the risk of population extinction under the 'Pessimistic' outlook was much higher, with a sustained climatically-unfavourable period of 11 years leading to extirpation.