Aad C. Smaal

Responses of Mytilus edulis L. to varying food concentrations: testing EMMY, an ecophysiological model

In this paper a complex ecophysiological model is presented, which aims to simulate individual growth and reproduction of Mytilus edulis L. The model includes feedback mechanisms in the acquisition and metabolism of natural food sources and partitioning of carbon and nitrogen to the internal state variables: somatic tissue, storage, organic shell matrix, blood and gametes before and after spawning. The model was calibrated with statistical distributions for 38 parameters. The resulting a posteriori parameter sets were applied in a validation procedure. First inputs of one system were used to produce model outcomes with uncertainty bands in order to compare these with system observations not used for calibration. In a second validation step, the model was run with inputs of two different ecosystems. The results of this step were promising, but no acceptable growth could be predicted for the system with low seston and food concentrations.

Filtration and absorption of Phaeocystis cf. globosa by the mussel Mytilus edulis L.

Mussels, Mytilus edulis L., were exposed to natural seawater, supplemented with variable amounts of Phaeocystis cf. globosa solitary cells and colonies and to a reference diet consisting of natural seawater, supplemented with Phaeodactylum tricornutum. Clearance rate showed a linear decrease with the amount of colonies present, expressed as total ‘circular equivalent surface area’. Extension of the exposure period for 28 h resulted in a further decreased clearance rate. Recovery of the clearance rate began immediately after exposure to untreated natural seawater. In the absence of colonies, there was a moderate clearance rate reduction. Filtration at reduced clearance rates was not compensated by increased particle concentrations and no adaptative response to the diet was observed. In an earlier experiment, we observed that the absorption efficiency of ingested Phaeocystis diet was not significantly different from the reference diet. It is concluded that the adverse effects of Phaeocystis blooms are mainly determined by the total colony surface area. There is no adaptation over time to diets with colonies. Once ingested, Phaeocystis is absorbed normally and has significance as food.

The response of phytoplankton communities to phosphorus input reduction in mesocosm experiments

This study examines the effects of a reduction in the phosphorus input (in comparison to field levels) on natural coastal phytoplankton communities. Experiments were conducted in four land-based mesocosms consisting of 3 m(3) black polyethylene tanks. Light climate and mixing regime reproduced conditions in Dutch coastal waters, temperature was maintained within 2 degrees C of the ambient seawater temperature. Phytoplankton species succession and primary production were studied in two mesocosm experiments of 5 and 4 weeks in April and July 1993. In both experiments, inorganic nutrients were continuously added to four mesocosms. N (nitrogen) and Si (silicon) inputs were identical for the four mesocosms and resembled the inputs to the Dutch coastal zone. The phosphorus (P) input established a N/P ratio gradient among the mesocosms: 16, 32, 64 and 128. The first experiment was carried out during the spring diatom bloom. The initial diatom dominance was maintained in the mesocosms with reduced phosphorus loads. In the mesocosm with the highest P input, flagellates developed to the detriment of the diatoms at the end of the experiment in parallel with the establishment of a silicate limitation. The second experiment coincided with the spring/summer phytoplankton bloom dominated by the colony of Phaeocystis sp. Diatoms outcompeted Phaeocystis sp. in the mesocosms with reduced phosphorus inputs (N/P = 32, 64, 128). In both experiments, maximum primary production rates were observed in the mesocosm with the highest P input and decreased with the phosphorus gradient. It was concluded that a reduction of the phosphorus loads to eutrophicated coastal waters determined a proportional reduction of the primary production and stimulated the dominance of diatoms over flagellates. [KEYWORDS: primary production; species succession; zooplankton grazing; mesocosm North-sea; plankton communities; eutrophication; competition; nutrient]

Seasonal variation in the filtration rates of a semi-natural mussel bed in relation to seston composition 1☆

Abstract In September 1988 a mussel bed, with a biomass of 1362 g ash-free dry weight· m−2, was created in a concrete tank with a continuous supply of natural seawater. Measurements of particulate matter uptake by the mussel bed were carried out monthly from December 1988 till December 1989. Fluxes of suspended particulate matter, particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphorus (POP) and chlorophyll-a, were significantly correlated with water column concentrations. A control experiment showed that sedimentation did not affect observed fluxes. Clearance rates of the mussel bed varied between 0.4 and 2.7 m3·m−2·h−1. Seasonal variation in the filtration activity of the mussel bed could be attributed to changes in the composition of the suspended particulate matter: individual clearance rates of the mussels were reduced in the period April–June, which coincided with a bloom of Phaeocystis sp. Moreover, clearance rates decreased with increasing SPM concentrations and showed a positive correlation with chlorophyll-a.

Does the Introduction of the Pacific Oyster Crassostrea gigas Lead to Species Shifts in the Wadden Sea

Over centuries dramatic changes have occurred in the species composition of the Wadden Sea, a shallow coastal sea bordering the North Sea. Natural dynamics as well as direct and indirect anthropogenic influences have resulted in the introduction and the disappearance of important benthic populations. Historic records and extensive surveys show large variability in benthic suspension-feeder stocks. Infaunal species like the cockle (Cerastoderma edule) are extremely variable over time and space, hence show a typical resilient response. Mussel (Mytilus edulis) beds seem to be more stable over time. Once lost, mussel beds need more time to re-establish bed structures. It is hypothesized that infaunal populations have a high resilience, while epifauna species are characterized by resistance to changes as they form structures like reefs or beds. On the basis of this hypothesis the consequences of new introductions can be evaluated. It can be expected that the recent introduction of the resistant reef-building epifaunal Pacific oyster Crassostrea gigas, will lead to shifts in benthic suspension-feeder populations and eventually will develop a new stable state for the Wadden Sea that potentially offers less food for birds. This situation may deviate considerably from the actual nature conservation objectives that focus on the role of the Wadden Sea as one of Europes most important wetlands for migratory bird populations.

The ecophysiological response of mussels (Mytilus edulis) in mesocosms to a range of inorganic nutrient loads: simulations with the model EMMY

EMMY is an ecophysiological model of the growth and reproduction of a single mussel (Mytilus edulis L.). It contains feedback loops in the uptake and metabolism of food and in the partitioning of carbon and nitrogen to the internal state variables somatic tissue, storage, organic shell matrix and gametes. In this paper EMMY is used to simulate individual mussel growth in a series of mesocosm experiments with different inorganic nutrient loads (N and P). The experiments explore the impact of eutrophication reduction scenarios on mussel growth under defined and controlled conditions.In earlier studies EMMY was calibrated using expert knowledge on growth and reproduction during a period of 5 years. The resulting calibrated model was validated for system inputs and observations of three ecosystems with significantly different food and silt concentrations. EMMY reproduced the mussel growth sufficiently accurate in ecosystems with moderate or high food concentrations. In this study EMMY was adapted in order to cope with low food concentrations, then recalibrated (using the original calibration data and procedure) and applied without further calibration to 3 replicated mesocosm experiments. The EMMY simulations in this study show the ecophysiological response of mussels to different food (phytoplankton and detritus) concentrations. It is concluded that the mussels can adapt to significantly reduced food concentrations, due to inorganic nutrient load reduction, and still maintain growth.

Effects of different N- and P-loading on primary and secondary production in an experimental marine ecosystem

The effects of nutrient loading on phytoplankton, zooplankton and macrozoobenthos in experimental ecosystems was studied in a 7-month experiment. The mesocosms were designed to mimic the major physical characteristics (irradiance, temperature, mixing) of the Dutch coastal zone in the river Rhine plume. Three different nutrient loading scenarios were used, representing present and future conditions. The level of the spring phytoplankton bloom was determined by phosphorus loading, whereas during summer the nitrogen loading determined phytoplankton biomass. The differences in nutrient loading did not result in shifts in phytoplankton species composition. With exception of the early phase of the spring bloom, diatoms dominated phytoplankton biomass in all nutrient treatments. This was ascribed to microzooplankton grazing on smaller algal species. Microzooplankton biomass showed a positive correlation with primary production, and also significant differences between nutrient treatments. Copepod development was limited, probably due to competition with microzooplankton and predation by benthic fauna. Macrobenthos biomass correlated with primary production, and was lower in the lowest nutrient treatment.

Physiological response of Spisula subtruncata (da Costa, 1778) to different seston quantity and quality

Individuals of the bivalve Spisula subtruncata were fed a mixed diet comprising of sea water enriched with the diatom Phaeodactylum tricornutum and ashed silt within a range of concentrations, simulating natural conditions above pseudofaeces threshold. The designed ranges for total particulate matter were between 10 and 30 mg l−1and organic content of seston 15–40%. Filtration rate, rejection rate, ingestion rate and absorption rate were measured at those different conditions. Filtration rate and rejection rate were significantly correlated to total particulate matter and percentage of organic matter, with higher rates at higher values of total particulate matter and lower values of percentage organic matter. Ingestion rate was maintained at similar levels in all the treatments and organic enrichment of the ingested food occurred due to preingestive selection of the filtered material. A differential absorption rate occurred at different levels of organic matter in the diet with high rates at high values of the organic content of the diet. S. subtruncata showed different physiological responses to changes of the food conditions: (1) Increase of pseudofaeces production at increasing levels of particulate matter, (2) preingestive selection of organic material which enriched the organic fraction of ingested food, (3) stabilized ingestion rate and (4) increase of the absorption rate at high organic levels of the seston.

Underwater Towed Video: A Novel Method to Estimate Densities of Queen Conch (Lobatus gigas; Strombidae) Across Its Depth Range

ABSTRACT Queen conch (Lobatus gigas) populations living deeper than 20 m are rarely studied, because of the limitations of conventional survey methods using divers [i.e., belt transect (BT), towed-diver]. Acrucial management goal for conch populations is to maintain adult densities at adequate levels to ensure reproduction, which is highly density dependent. Therefore, accurate estimates of adult conch densities, both in shallow and deep areas, are essential. The rapid technical progress of video systems has made it possible to develop new cost-effective ecological sampling tools, which can be used to survey areas previously hardly accessible. A lightweight towed video array was used, which was able to survey adult conch throughout the species entire depth range (ca. 0–60 m depth), in a safe and efficient manner. The towed video method (TVM) was compared with a conventional BT method using scuba divers, in its ability to identify adult live and dead conch. A series of intercalibration transects was conducted in a high-complexity (HC) and in a low-complexity (LC) habitat by having the towed video followed by a diver conducting a concurrent standard BT, covering the exact same surface area as the towed video. In both the HC and LC habitat, adult live queen conch had similar counts with both methods. Adult dead conch were not mistaken for live conch but were significantly underestimated with the towed video compared with the BT. The results validate the use of TVM as a reliable sampling tool to estimate densities of live adult conch in both HC and LC habitats throughout the species depth range.

Culturing Chaetoceros muelleri using simplified media with different N sources: effects on production and lipid content

Land-based bivalve aquaculture depends on large-scale cultures of live microalgae for food. The intensity of large-scale microalgal production is important for cost-effectiveness. Using Walne’s medium as the control, simplified media containing nitrogen, phosphorus, silica, iron, manganese and vitamins were designed to determine the impact of nitrogen source and molar N:P ratio (sodium nitrate, NO3 9:1, ammonium chloride, NH4 9:1 and NH4 25:1) on growth, dry-weight biomass, culture longevity and lipid content of Chaetoceros muelleri, a diatom commonly used in shellfish aquaculture. During the exponential phase (day 6), dry-weight production in simplified media was similar to controls, indicating that this microalga can grow successfully on simplified media and use ammonium as the nitrogen source. The cultures grown on nitrate or ammonium differed in their time-course. Low nitrogen concentration in cultures grown with nitrate caused the collapse of these cultures within 11–13 days, after a short stationary phase. Cultures grown with ammonium had a longer stationary phase and were still alive on day 20, in spite of the low nitrogen concentrations observed after day 13 in cultures grown with NH4 9:1. During stationary phase (day 18) there was an increase in lipid content of algae under conditions of low nitrogen availability (NH4 9:1) and extended low phosphorus availability (NH4 25:1). Considering dry weight production, culture longevity, nutrient efficiency and lipid composition, simplified media containing ammonium, phosphorus, silica, iron, manganese and vitamins are a viable and profitable choice for batch culture of C. muelleri. In the exponential phase, the simplified medium NH4 9:1 was as effective as the control. Overall, both of the simplified media using ammonium are effective and suitable, depending on the purpose of the cultures and whether lipid contents (NH4 9:1), dry weight biomass (NH4 25:1) or nitrogen input and output (NH4 9:1) are desired.