Douglas C. Speirs

POPULATION PERSISTENCE IN RIVERS AND ESTUARIES

A wide variety of organisms inhabit streams, rivers, and estuaries where they are continually subjected to downstream drift. It is well known that when this is the only transport process, extinction is inevitable (the “drift paradox”). Using a series of analytical and numerical models, representing a range of hydrodynamic scenarios, we demonstrate that the action of diffusive dispersal can permit persistence in an advective environment. The mechanism underlying this phenomenon is that diffusive dispersal can allow a proportion of the population to reproduce close to their natal location. For well- and poorly mixed non-tidal systems we establish approximate analytic conditions for diffusion-mediated persistence both throughout the water column and in a benthic boundary layer. Although tidal forcing results in residual landward flow near the base of the water column, we find that this has little effect on persistence, which is respectably approximated by our analytic results. We apply these analytic results to four hydrodynamically disparate systems: a stream (Broadstone Stream [UK]), a river (Christiana Creek [USA]), a shallow estuary (Ythan [UK]) and a deep fast-flowing estuary (Saco River [USA]). Using parameters derived from published studies we examine the persistence of a number of real and hypothetical organisms in these systems and identify those for which diffusively mediated persistence is a realistic possibility. We note that such persistence is only likely when advection is low or horizontal dispersal is high.

Understanding patterns and processes in models of trophic cascades

Climate fluctuations and human exploitation are causing global changes in nutrient enrichment of terrestrial and aquatic ecosystems and declining abundances of apex predators. The resulting trophic cascades have had profound effects on food webs, leading to significant economic and societal consequences. However, the strength of cascades–that is the extent to which a disturbance is diminished as it propagates through a food web–varies widely between ecosystems, and there is no formal theory as to why this should be so. Some food chain models reproduce cascade effects seen in nature, but to what extent is this dependent on their formulation? We show that inclusion of processes represented mathematically as density-dependent regulation of either consumer uptake or mortality rates is necessary for the generation of realistic ‘top-down’ cascades in simple food chain models. Realistically modelled ‘bottom-up’ cascades, caused by changing nutrient input, are also dependent on the inclusion of density dependence, but especially on mortality regulation as a caricature of, e.g. disease and parasite dynamics or intraguild predation. We show that our conclusions, based on simple food chains, transfer to a more complex marine food web model in which cascades are induced by varying river nutrient inputs or fish harvesting rates.

Changes in species diversity and size composition in the Firth of Clyde demersal fish community (1927–2009)

Following the repeal in 1962 of a long-standing ban on trawling, yields of demersal fish from the Firth of Clyde, southwest Scotland, increased to a maximum in 1973 and then declined until the directed fishery effectively ceased in the early 2000s. Since then, the only landings of demersal fish from the Firth have been by-catch in the Norway lobster fishery. We analysed changes in biomass density, species diversity and length structure of the demersal fish community between 1927 and 2009 from scientific trawl surveys, and related these to the fishery harvesting rate. As yields collapsed, the community transformed from a state in which biomass was distributed across numerous species (high species evenness) and large maximum length taxa were common, to one in which 90 per cent of the biomass was vested in one species (whiting), and both large individuals and large maximum length species were rare. Species evenness recovered quickly once the directed fishery ceased, but 10 years later, the community was still deficient in large individuals. The changes partly reflected events at a larger regional scale but were more extreme. The lag in response with respect to fishing has implications for attempts at managing a restoration of the ecosystem.

Cascading ecological effects of eliminating fishery discards

Discarding by fisheries is perceived as contrary to responsible harvesting. Legislation seeking to end the practice is being introduced in many jurisdictions. However, discarded fish are food for a range of scavenging species; so, ending discarding may have ecological consequences. Here we investigate the sensitivity of ecological effects to discarding policies using an ecosystem model of the North Sea—a region where 30–40% of trawled fish catch is currently discarded. We show that landing the entire catch while fishing as usual has conservation penalties for seabirds, marine mammals and seabed fauna, and no benefit to fish stocks. However, combining landing obligations with changes in fishing practices to limit the capture of unwanted fish results in trophic cascades that can benefit birds, mammals and most fish stocks. Our results highlight the importance of considering the broader ecosystem consequences of fishery management policy, since species interactions may dissipate or negate intended benefits.

The swimming behaviour and distribution of Neomysis integer in relation to tidal flow

The behaviour and distribution of the estuarine mysid shrimp, Neomysis integer, were investigated in relation to a variety of flow regimes in the field and in the laboratory. In the field, Neomysis aggregated in low-flow areas, such as in the lee of rocks and macroalgal clumps. Over the low-tide period and during ascending and high-tide periods, individuals were concentrated in a band toward the moving tide edge where flows are typically low. Experiments performed using a laboratory flume demonstrated a clear response of Neomysis to flow, the proportion of individuals above the bed declining with increasing flow. At flows of up to 10 cm s−1, 90–95% of individuals were able to maintain position on the bed of the flume, but at flows greater than this, Neomysis were unable to maintain position. When provided with a choice of water depths in a laboratory static tank (zero flow), there was little evidence of depth-seeking behaviour. We suggest that avoidance of areas of high flow is a key factor determining the local distribution of Neomysis.

Why do shallow-water predators migrate? Strategic models and empirical evidence from an estuarine mysid

We investigated factors potentially affecting tidal migrations over the littoral zone by invertebrate predators via a combination of strategic modelling, field observations and laboratory experiments using the mysid Neomysis integer. The models predict the distribution of individuals over the immersed intertidal region under the three different scenarios of diffusive movement, movement up a gradient of prey abundance, and movement towards a specific water depth. We reject the diffusive spread hypothesis since the predicted changes in spatial patterns are qualitatively inconsistent with those of density estimates over the tidal cycle in the field. The foraging hypothesis was consistent with the field samples only if there was a consistent upshore food gradient. Gut contents analysis showed meiofaunal prey were rare, but that organic detritus was the main dietary component. Sediment samples indicated some evidence of an organic matter gradient, but in the laboratory, Neomysis showed no preference for sediment with high organic content. We therefore rejected the foraging hypothesis. The depth-seeking model was not rejected, but laboratory experiments involving responses to various predators of Neomysis provided only weak evidence that depth-seeking was a result of predator avoidance. We hypothesise that the proximate mechanism is most likely to involve behavioural responses to flow conditions.

Spatial modelling of Calanus finmarchicus and Calanus helgolandicus : parameter differences explain differences in biogeography

The North Atlantic copepods Calanus finmarchicus and C. helgolandicus are moving north in response to rising temperatures. Understanding the drivers of their relative geographic distributions is required in order to anticipate future changes. To explore this, we created a new spatially explicit stage-structured model of their populations throughout the North Atlantic. Recent advances in understanding Calanus biology, including U-shaped relationships between growth and fecundity and temperature, and a new model of diapause duration are incorporated in the model. Equations were identical for both species, but some parameters were species-specific. The model was parameterized using Continuous Plankton Recorder Survey data and tested using time series of abundance and fecundity. The geographic distributions of both species were reproduced by assuming that only known interspecific differences and a difference in the temperature influence on mortality exist. We show that differences in diapause capability are not necessary to explain why C. helgolandicus is restricted to the continental shelf. Smaller body size and higher overwinter temperatures likely make true diapause implausible for C. helgolandicus. Known differences were incapable of explaining why only C. helgolandicus exists southwest of the British Isles. Further, the fecundity of C. helgolandicus in the English Channel is much lower than we predict. We hypothesize that food quality is a key influence on the population dynamics of these species. The modelling framework presented can potentially be extended to further Calanus species.

Projected impacts of 21st century climate change on diapause in Calanus finmarchicus

Diapause plays a key role in the life cycle of high latitude zooplankton. During diapause, animals avoid starving in winter by living in deep waters where metabolism is lower and met by lipid reserves. Global warming is therefore expected to shorten the maximum potential diapause duration by increasing metabolic rates and by reducing body size and lipid reserves. This will alter the phenology of zooplankton, impact higher trophic levels and disrupt biological carbon pumps. Here, we project the impacts of climate change on the key North Atlantic copepod Calanus finmarchicus under IPCC RCP 8.5. Potential diapause duration is modelled in relation to body size and overwintering temperature. The projections show pronounced geographic variations. Potential diapause duration reduces by more than 30% in the Western Atlantic, whereas in the key overwintering centre of the Norwegian Sea it changes only marginally. Surface temperature rises, which reduce body size and lipid reserves, will have a similar impact to deep-water changes on diapause in many regions. Because deep-water warming lags that at the surface, animals in the Labrador Sea could offset warming impacts by diapausing in deeper waters. However, the ability to control diapause depth may be limited.

Sea-Age Variation in Maiden Atlantic Salmon Spawners: Phenotypic Plasticity or Genetic Polymorphism?

Atlantic salmon exhibit a partially heritable polymorphism in which the morphs are distinguished by the duration and location of the sea-phase of their life-cycle. These morphs co-occur, albeit in characteristically different proportions, in most Scottish rivers and in both the spring and autumn spawner runs; early running fish being generally associated with upland spawning locations while late running fish are associated with lowland spawning. Thus, differences in riverine and marine environment appear to be linked to differences in the relative abundance of the morphs, rather than to the specific morph which is optimally adapted. In this paper, we report a model-based synthetic study aimed at understanding the key dynamic elements which determine the long-term stability of this polymorphism, and thus determine the relative abundance of the various sea-age morphs. Given the recent accumulation of evidence for a genetic basis for the polymorphism, we argue that the key dynamic mechanism which equalises the realized fitness of the sea-age morphs must be one or more morph-specific density dependencies in the riverine phase of the life-history. We explore a number of specific mechanisms, firmly based in known salmon biology, by which such morph-specific density dependence could occur and investigate the robustness of the co-existence which they imply. We conclude that the co-occurrence of multiple sea-age morphs of Atlantic salmon in Scottish rivers is a stable genetic polymorphism, maintained by some combination of physical separation and asymmetric competition between spawners of different morphs or the riverine stages of their offspring or both.

Effect of collisions on amplification of laser beams by Brillouin scattering in plasmas

We report on particle in cell simulations of energy transfer between a laser pump beam and a counter-propagating seed beam using the Brillouin scattering process in uniform plasma including collisions. The results presented show that the ion acoustic waves excited through naturally occurring Brillouin scattering of the pump field are preferentially damped without affecting the driven Brillouin scattering process resulting from the beating of the pump and seed fields together. We find that collisions, including the effects of Landau damping, allow for a more efficient transfer of energy between the laser beams, and a significant reduction in the amount of seed pre-pulse produced.