The number of heatwaves in the Netherlands is on the rise due to climate change. Apparently, this is something cockles do not handle well: there were mass mortalities among cockles in the summers of 2018, 2019 and 2020 during heatwaves. In periods of extreme heat, the mortality rate among cockles in the Wadden Sea and Oosterschelde varied between 60 and 96 per cent, depending on age and region. How precisely does temperature affect the quality of benthic life and food supply for birds? Wageningen Marine Research scientists conducted soil temperature measurements commissioned by the Ministry of Agriculture, Nature and Food Quality (Dutch acronym LNV). They conclude that extreme mortality rates are the result of high temperatures on sandbanks during low tide, where cockles are buried in the sediment.
Researcher Lisanne van den Bogaart explains that the shellfish expert team is not yet able to predict how the cockle population will respond to rising temperatures and even more extremes, such as an increased number of heatwaves. ‘The next decade may well show excellent spat settlements or the cockles may adapt to the changing circumstances. Another option is that the species will be severely impacted and may even be replaced by the Manilla clam, which has been profiting from rising temperatures in the waters of the delta over the past years’, Van den Bogaart states.
Various measuring locations in the Oosterschelde
The research was conducted at three different locations in the Oosterschelde, in different sub-regions: near the Oesterdam, in the central part of the muddy plains of the Dortsman, and at the Roggenplaat. Samples were taken at different times during low tide. In 2020 sediment samples were taken to calculate the density of the cockles on these locations in May and at the start of September. The mortality rate of three age categories was determined. The temperature measurements were taken during these months using temperature loggers (situated at ground level and at three and ten centimetres below ground level). Besides, light loggers and pressure were placed to measure the suns radiation and water depth.
Location and sediment type determine the impact of heatwaves on cockles
Temperature measurements showed that the sediment temperature reached high levels during the heatwave of 5 to 10 August 2020. During these thirteen days, tropical temperatures above 30°C were recorded on nine days. On the Roggenplaat, temperatures of 35°C were recorded at a depth of three centimetres. Although the nights were cooler, the temperature remained well above 20°C. This means the cockles suffered from extreme temperature changes.
The lowest mortality rate was recorded on the Roggenplaat (62%), while the Oesterdam and Dortsman showed an extremely high mortality rate of 94% and 92%, respectively.
The difference appears to be due to the type of sediment. The muddy Oesterdam did not heat up as rapidly as the Roggenplaat but also retained the heat for longer during the night. Sandy areas such as the Roggenplaat have better drainage, which means they heat up faster but also cool down more easily. In areas that were submerged, the temperatures did not reach peak levels due to the buffering effect of the cooler seawater. Water also mitigates the effects of the suns radiation. This prevents the sediment from heating up too much but also from cooling down during the night. In elevated areas, where the low tide leaves the sand dry for a longer period, there is more time for the sand to heat up, resulting in higher sediment temperatures.
The recorded mortality rates appeared to be linked to the number of hours the temperature dropped below the 23°C threshold during the night. On the Roggenplaat, this occurred for relatively longer than at the other locations. The researchers suspect the soil permeability played an important role, as this allows the cooler seawater to cool down the sand faster during high tide. This gave the cockles time to recover during the night from the day’s high temperatures, ultimately resulting in lower mortality.