Contrasting timing of life stages across latitudes - a case study of a marine forest-forming species

In this study we combine field demographic surveys with controlled culture experiments, to understand if temperature might be the factor leading to differences in life expectancy and seasonality across different latitudinal ranges in the kelp Saccorhiza polyschides. This species overwinters as a macroscopic sporophyte in geographically central populations, like Brittany (northwestern France), while in northern Portugal, the southern range of its distribution, only microscopic forms are present in winter. Our results did not support temperature as the cause of the different life histories observed in central and southern ranges. We found that sporophyte recruitment occurs year-round in Brittany while farther south it is limited to spring. Our results show that this seasonal recruitment is not due to winter temperature, as zoospores were able to develop between 5 and 20 degrees C with an optimum at 10-15 degrees C, placing the Portuguese winter temperatures within the optimum range. In addition, we showed that the synchronized release of zoospores in early autumn in the southern range does not explain the absence of recruits throughout winter, as these took a maximum of 62 days, at 5 degrees C, to develop into a sporophyte visible in the field (5 cm) and only 20 days at 10 degrees C. An alternative hypothesis that could explain the different life histories is the seasonality of high seawater nutrient levels occurring in the southern but not in the central-northern range, where permanently higher nutrient concentrations might allow sporophytes to occur in any season. This hypothesis is supported by the coincidence of macroscopic sporophytes with the seasonal upwelling period in the southern range, but it requires further testing.