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  • Current Trends in Geomathematics.
  • School of Earth and Climate Sciences.
  • Seasonal variation in the diet of estuarine bivalves.

Worldwide, geomorphological classifications of estuaries are often used to guide the design of monitoring programs and management strategies. However, if classifications do not reflect biotic patterns, the effectiveness of monitoring and management is potentially reduced.

In this study, we consider the effectiveness of one classification scheme in describing biotic patterns by examining and comparing spatial variation of macrofaunal assemblages and their relationship with the environment in 12 estuaries of 2 geomorphological types mesotidal river dominated and permanently open barrier estuaries. Estuaries were sampled at three locations upper, mid and lower for macroinvertebrates and environmental characteristics. Differences in macrofaunal assemblages occurred between the estuary types at the lower and mid locations, but not the upper.

Long-term estuarine variability and associated biological response | SpringerLink

Similarities in the upper locations were related to sediment, whereas at the mid and lower locations differences were linked to salinity, dissolved oxygen concentrations and seagrass area. Within estuaries, location effects were definitive and unique within each estuary type, correlating to changes in sediment particle size, nitrogen concentration, microphytobenthos and percentage organic carbon.

These results suggest that estuarine classification based on physical attributes alone does have the potential to capture important biological attributes if the biological scales of variability within these systems are well understood. For M. The slopes of these relationships were not significant for the other three estuarine bivalve species examined Fig 6, S1 Table.

For four bivalve species C. For the edible cockle C. For the Pacific oyster M. The slope of these relationship was not significant for L. For C. Also for M. In March, the relatively large isotopic niche of L. In June, the sizes and overlaps in isotopic niches appeared to be smaller than in March, with the smallest overlap being between that of L.

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In September, the overlap of the relatively large isotopic niche of L. In December, there was no overlap between the isotopic niches of C. At the Balgzand tidal flats, the highest availabilities of the three primary food sources occurred at different times of the year. Marine phytoplankton had the highest availability in the spring, microphytobenthos peaked in summer, while freshwater phytoplankton had peak values in the autumn.

In general, highest values of freshwater that enter this system are found in winter up to m 3 s -1 and lowest in summer with no water supply at all on some days [ 68 ]. In , the supply of freshwater algae to the Wadden Sea was highest in autumn as a result of a combination of high freshwater discharge and the highest chlorophyll a concentration in Lake IJssel and lowest in April, mostly due to low levels of water discharge in that month. Also the dead organic matter detritus can be a valuable food source for estuarine bivalves [ 40 — 43 ].

Because the local availability of these resources might also vary in time e. Sampling should then preferably be done close to the sediment within the full tidal window when bivalves have direct access to their food. Ideally the isotopic signal of pure phytoplankton samples would be the best solution to exclude the effect of other particulate matter on the signal.

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However, this type of sampling is time consuming and might not be a realistic approach. An alternative would be the analysis of compound specific isotopic composition e. However, several of the averages of food sources in this study all freshwater SPOM and microphytobenthos in December were based upon single observations, leading to some uncertainties regarding the stable isotope values of the sources.

Explaining Variability

Also within the mixing model correlations of up to 0. Such large correlations indicated that the model cannot discern between the two sources, which means that if one source is being consumed at the top of its probability range, then the other is likely to be at the bottom of its probability range, and vice versa [ 73 ]. During this study, two species Abra tenuis , Scrobicularia plana and one period December were undersampled and are, therefore, not further considered. In March, June and September , the obligate suspension-feeding bivalves Cerastoderma edule , Magallana gigas , Mya arenaria and Mytilus edulis appeared to have predominantly utilized freshwater SPOM as their main food source.

Only Limecola balthica , a bivalve species that can switch between deposit- and suspension-feeding [ 29 , 74 ], appeared to have fed mainly on estuarine SPOM during spring. In June and September , this Baltic tellin appears to have preferred microphytobenthos, most probably gathered by means of deposit-feeding. It should be noted, however, that most correlations between food sources were high, indicating that the models could not discern very well between these resources [ 73 ].

These outcomes are, nevertheless, in concordance with previous findings that freshwater SPOM can be an important food source for estuarine obligate suspension-feeding bivalves [ 40 — 43 ] and that main food sources of estuarine facultative deposit-feeding bivalves can vary within the year [ 28 ]. The results of the mixing model are also highly depending on the trophic discrimination factor. Within this study we used values that were measured by Dubois [ 63 ], because these values were based upon a feeding experiment on two estuarine bivalve species also included in our research M.

Using other values for trophic discrimination factors did, however, not change our main conclusion that all species feed on a mixture of different sources and that the freshwater SPOM is a relevant part of their diet. As was observed for edible cockles C. In general, relationships between isotopic signals and size might also be explained by differences in growth rates, with juveniles generally growing faster than adults [ 75 ]. Based on the data presented here, this is all very speculative, but this question definitely deserves some further attention. For Baltic tellins L. The shift in the soft-shelled clam M.

If the threshold where this species stops feeding increases with its size, then this bivalve might prefer to feed more and more during food-rich waters caused by discharge outflows during its life time. Mussels frequently change position and orientation during their life time e. If small mussels settle, on average, at the top of a mussel bed and then gradually move down to lower locations as new mussels and oysters settle on top of them, they would get less and less access to the relatively light freshwater containing freshwater SPOM and more and more to the denser saline waters containing marine SPOM.

Estuarine bivalves such as C. If the threshold of edible zooplankton increases with shell size, then it is expected that the proportion of primary consumers increases during the life time of bivalves. Within the estuarine bivalve community, the interspecific niche overlap was highest in March. This seasonal variation in niche overlap was partly due to the variation in species-specific niche breadths. Such seasonal variation might indicate that the bivalves were relying on a wider array of resources in March than in June, but also that the bivalves were more food-limited in spring than in summer [ 75 ].

Although food limitation is not very likely because the relatively high niche breadth is mainly found within the obligate suspension-feeders during the phytoplankton spring bloom, this possible cause cannot be excluded. More probable, the bivalves took advantage of the high availability of the same food source at that time. Interspecific competition for food was more likely to occur during summer, when marine phytoplankton densities were low as the result of low nutrient concentrations and the supply of freshwater phytoplankton was low due to low inflows from Lake IJssel.

Using macrofaunal communities to inform estuarine classification

During that time, microphytobenthos is probably the main potential food source for estuarine bivalves, which is in line with the findings by Christianen et al. For obligate suspension-feeding bivalves, their food source might only come available during high wind speeds when benthic algae are resuspended into the water column [ 25 ]. Our results indicate that freshwater algae may contribute to the food supply of estuarine bivalves in the study area, and that diets of bivalves vary with season and shell length. Although not all parts of the Wadden Sea receive as much freshwater import as the western Wadden Sea, the estuaries of large rivers such as Ems, Elbe and Weser show comparable freshwater inflows [ 68 ].

Diet studies should evaluate all food sources utilized by temperate estuarine bivalves and sampling should include all possible resources and be in close proximity to the feeding space of these bivalves. Seasonal mean, sd and number of samples n of stable isotope values of the different bivalve species collected at two stations on the Balgzand tidal flats in , empty fields indicate that no samples have been collected, trophic guild based on Kamermans [ 18 ]. We would like to thank all the volunteers and assistants, as well as the Crew of the RV Stern for the support in collecting and sorting the samples for this study and the support in the Lab.

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Click through the PLOS taxonomy to find articles in your field. Abstract Estuarine food webs are generally considered to be supported by marine pelagic and benthic primary producers and by the import of dead organic matter from the open sea. Introduction Coastal ecosystems and their food webs are under the influence of a variety of tidal and seasonally fluctuating factors in environmental conditions e.

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