The role of diapausing eggs in zooplankton biodiversity and dispersal

Résumé

Inland freshwater bodies host a wide variety of aquatic invertebrates. The ponds and lagoons located in Mediterranean areas are subject to a high degree of temporality and environmental unpredictability that affects seasonal and interannual changes in zooplankton communities. Under these circumstances, many zooplankton species during their life cycle may enter the diapause stage when environmental conditions are adverse, such as adults or through the production of diapausing eggs, remain viable for long periods of time and end the diapause period when conditions are again favorable. However, not all diapausing eggs hatch before the stimuli that indicate the return of favorable conditions, leaving a small fraction of diapausing eggs in sediment. This accumulation of diapausing eggs from different cycles is known as diapausing egg bank. Therefore, its characterization is fundamental, as well as knowing its capacity of survival and dispersion. To evaluate the efficiency of zooplankton dispersal, it is important to note that the dispersion may be limited by the viability of diapausing eggs. The identification of appropriate signals for hatching of diapausing eggs is of crucial importance in assessing such viability. The zooplankton dispersal capability may regulate the population dynamics and aquatic community structure and play a key role in the colonization of new water bodies. Zooplankton is potentially dispersed overland by abiotic vectors like wind, rain and water flow, or by organisms like waterfowls or insects. Although many studies have studied the dispersal vectors, quantitative measures of zooplankton dispersal have been rare and mainly focused on waterfowls. Unfortunately, little is known about dispersal rates by wind or rain due to the difficult to measure it. In Chapter 2 we quantify the zooplankton propagules dispersal by wind and rain using two atmospheric deposition collectors, which permit to collect samples from dry atmosphere deposition and wet atmosphere deposition. Two of these collectors were placed beside the lakes Dulce and Santa Olalla, eutrophic lakes in Doñana Natural Park, and a third one was located close to the lakes Tinaja and Morenilla, oligotrophic lakes in the Ruidera Natural Park. As the collectors located in Doñana gathered more quantity of zooplankton propagules, we built two nylon wind socks beside these collectors for measuring the particulates blowing in the wind. The results shown differences in the number and the species propagules collected between the lakes of different location. The higher abundance of dormant propagules was registered in the wind socks, but the presence of them in both collectors suggested the importance of the aerial dispersal. Our results suggest the relative importance of wind and rain as dispersal mechanisms of the zooplankton dormant propagules and their impact on the biodiversity of aquatic systems. Waterfowl that inhabit ponds and shallow lagoons are an important vector of dispersion of dispersal propagules, being transported in the feathers or lodged in the digestive tract, and in both cases without loss of viability. At present, all studies of dispersal of aquatic invertebrate diabetic propagules have focused on crustaceans, and only a few have considered rotifers, although collaterally, without a sufficient degree of taxonomic resolution and without evaluating the hatchability of eggs isolated from feces. In Chapter 3, cladoceran and rotifers dispersed diapausing eggs found in feces of waterfowl located in Dulce and Santa Olalla lakes in the Doñana National Park were isolated and identified. Their ability to survive under local environmental conditions was also evaluated. The results showed dispersion rates much higher than those recorded by the wind, especially by the group of rotifers. Therefore, the dispersal of diapausing eggs by birds can be considered a relevant dispersion mechanism, especially for rotifers. Salinity is becoming a serious threat to freshwater ecosystems due to global change, and it is known as a driving factor determining the presence and dominance of aquatic organisms. Zooplankton may produce resting eggs, which maintain its viability during long periods, being the egg banks research of interest to many biological phenomena, such as migration from the past, dispersal, temporal heterogeneity and biodiversity. In Chapter 4, the effects of salinity on zooplankton are mainly investigated by inducing hatching of zooplankton resting eggs. These eggs were isolated from surface sediment samples in lakes Santa Olalla and Dulce (Doñana National Park, Spain). To study factors affecting hatching rates, resting eggs were incubated at two temperatures (15 and 25 ºC) under a gradient of salinity (0.3-9 psu), which includes salinities that were usually observed in these lakes. Additionally, the temporal dynamic of zooplankton in the lakes during several years was studied. Our results showed that salinity concentration affect the hatching rates of rotifers, but this effect was species-specific and could be modified by temperature. Moreover, we observed differences in the hatching response of zooplankton regarding of the origin of the eggs and, in some cases, the day of hatching was positively related with salinity. These results will be discussed with the dynamic of zooplankton in these lakes. The potential contribution of the resting eggs bank to freshwater zooplankton resilience after increased salinity will also be considered. Water flows is considered a dispersion vector that facilitates the transport of both diapausing eggs and individuals which inhabit the water column, since the invertebrates of lentic systems do not usually resist water currents. Another important factor can be that like the dispersion by aquatic birds the transport takes place of a body of water to another body of water. In Chapter 5 we studied the diversity of the zooplankton community of Santa Olalla and Dulce in Doñana and Tinaja and Morenilla in Ruidera. The results showed that the most abundant species were also found in the largest number among the dispersed and identified diapausing eggs. Taking into account the point of connection in Ruidera, Hundimiento, lakes we verified that there were no differences in the composition of species between Tinaja and Morenilla, in contrast in the point of connection the community of zooplankton was slightly different. Possibly because Hundimiento acts as a river that connects both points with the homogenization of the zooplankton communities. In Doñana, between Santa Olalla and Dulce lakes, a flood connection occurs when the precipitation is very high. This phenomenon might act as a vector for zooplankton individuals dispersal. Nevertheless, dormant propagules dispersal by wind and waterbirds are more higher and frequent, and might determine the structure and homogenization of zooplankton species composition in Doñana lakes. Understanding the rates and mechanisms by which zooplankton moves between habitats is very important in understanding how zooplankton populations recover after natural or anthropogenic disturbances and interpret results obtained from experimental manipulations in aquatic systems. The results obtained show the relative importance of the dispersal mechanisms of zooplankton diapausing propagules and their possible impact on the biodiversity of aquatic systems. The consequences of dispersal of zooplankton on communities and diapausing egg banks can be assessed in terms of biodiversity conservation and aquatic systems manipulation. Predictions about recovery rates of disturbed communities after different disturbances may be more reliable by studying the mechanisms by which zooplankton organisms move between habitats.