Journal source: Jung AS, van der Veer HW, van der Meer MTJ, Philippart CJM (2019) Seasonal variation in the diet of estuarine bivalves. PLoS ONE 14(6): e0217003. https://doi.org/10.1371/journal.pone.0217003
At the intersection of land and sea, waves crash upon tidal flats, leaving behind foamy brine and sprays of mists into the salty air. Beyond the shoreline, small streams from inland lakes weave their way through the forest and surrounding land, determined to join the ocean. This inspiring blending of boundaries of two ecosystems is more than a pretty picture: waves carry marine phytoplankton, which includes many varieties of free-floating algae, while streams racing from the land introduce freshwater phytoplankton algae into coastal food webs.
Over the course of a year, coastal habitats experience changes in the availability of different types of resources that are carried in by marine and freshwater sources. From storms that stir up bottom sediments into the water to heavy rains that increase the pulse of stream-bound materials to the coasts, a seasonal buffet of food items becomes available to organisms residing in the coastal zones. These seasonal changes in available food sources may have important effects on coastal marine food webs.
The organisms most impacted by these fluxes are the primary consumers, the critters at the very bottom of the food chain that chow down on algae and other particles in the water. These organisms, in turn, are eaten by organisms higher up in the food chain, such as fish.
An important primary consumer of coastal food webs is the humble bivalve. Bivalves are a group of invertebrate animals that includes mussels and clams. These animals feed by means of a flexible siphon, or tube, which they use to vacuum up algae and other materials. Some species have siphons that are more suited for gathering food in the water column, while others have shorter siphons and feed mainly on the bottom sediments.
Changes in the available types of food could impact the diets and abundance of different bivalve species. This in turn could cause some species to decline in abundance and others to increase, affecting animals higher up in the food chain. However, the diets of coastal bivalves, and how they change over time with seasonal alterations of food sources, are not fully understood.
With various nutritional inputs entering the coasts at different times of the year, how do scientists find out what these animals are actually eating? A popular method that scientists use to look at diets of marine organisms is stable isotopes analysis. This method is based on the fact that different resources-phytoplankton, benthic, or bottom-growing algae, and particles from decaying organisms and plants-each have a distinct “signature” of carbon and nitrogen isotopes. These food resources will then affect the isotope signature of the organisms that consume them.
Jung and colleagues at Utrecht University, The Netherlands, investigated seasonal changes in the abundance of marine and freshwater food resources and the diets of different mussel species living in the Balgzand tidal flat area in the western part of the Wadden Sea. In March, June, September, and December, they collected seasonal samples for stable isotope analysis of the three major resource pools: marine phytoplankton, freshwater phytoplankton from small streams that enter the tidal flat, and benthic (bottom-growing on rocks and sediments) algae growing on the bottom surfaces of the tidal flat.
The scientists then did stable isotope analysis on the samples. To estimate how much of each food type was available during each season, the researchers measured biomass of the algae using chlorophyll, an algal pigment.
They also collected different bivalve species in the tidal flat to analyze them for stable isotope ratios and compare their signatures to those of the three food sources. The researchers were also interested whether diets of younger bivalves (with shorter shells) differed from older animals.
What did they find?
The authors found significant seasonal patterns in the availability of the three major food sources at Balgzand tidal flats. Marine phytoplankton were highest in spring, benthic algae was most abundant in summer, and freshwater phytoplankton peaked in autumn, which coincided with high freshwater stream flow into the tidal flat.
The stable isotope signatures of the bivalves changed throughout the seasons, following seasonal availability of marine and freshwater sources. The bivalves fed mainly on marine phytoplankton in March and June, which aligned with this resource having highest availability in spring, and mostly lived off freshwater phytoplankton in the winter.
The stable isotope signatures of young and old bivalves showed that their diets also varied with different stages of their life cycle. For the bivalve species Limecola balthica, juveniles fed mainly on benthic algae, while marine phytoplankton became more important to the diets of older individuals. Another species, the blue mussel M. edulis, fed mainly on marine algae during juvenile stages, and mature individuals ate almost exclusively freshwater phytoplankton.
The bigger picture: you are what and where you eat.
Coastal creatures enjoy varied diets, so food resources from the ocean, land, and freshwater can have profound effects on coastal food webs. Along with the natural seasonal variations in different food sources coming into the coasts, human activities may alter the quality of food. Urbanization and coastal development can introduce excess nutrients and pollutants into freshwater lakes and streams, which may end up in coastal zones to the dismay of mussels. This study highlights that management of coastal ecosystems needs to consider the health of both marine and freshwater sources, which are so important to coastal food webs.
Kate received her Ph.D. in Aquatic Ecology from the University of Notre Dame and she holds a Masters in Environmental Science & Biology from SUNY Brockport. She currently teaches at a small college in Indiana and is starting out her neophyte research career in aquatic community monitoring. Outside of lab and fieldwork, she enjoys running and kickboxing.