AbstractLovelock used a model of an idealised planet to demonstrate the stabilising effect of feedback on a population of daisies and environmental temperature. The equations he used represented a population model with only two steady states: no daisies present, or a stable population with monotonic damping. This paper investigates whether a similar result is obtained with a less stable population model. It was found that simple feedbacks, mediated by albedo and temperature, or by CO2 and the "greenhouse" effect, did not alter the basic properties of population models but could alter the stability regime within which a certain set of population parameters operated. Models of populations with discrete generations tended to be less stable with feedback while those with continuous growth were more stable. This confirms that Lovelock's result is not highly dependent on the type of population model. Adding environmental feedback to simple population models does not alter the basic properties of the models themselves but leads to a stabilisation of the whole system of environment plus population.
A numerical model of the tides in the neighbourhood of Portland Bill (Pingree & Maddock, 1977; Maddock & Pingree, 1978) indicated that there was some asymmetry in the flood and ebb tidal patterns due to the effects of the Earth's rotation. Thus, in addition to the generation of vorticity due to the bottom frictional torque which becomes large in the high-velocity shoaling region near the promontory, vorticity is generated as columns of water with planetary vorticity are stretched or squashed as they move across water depth contours. In order to examine the Portland Bill situation in more detail, a model was developed in plane polar coordinates (Pingree & Maddock, 1979). This model allowed the flow to be taken around the promontory without the numerical difficulties associated with corners in rectangular grid meshes and gives increased resolution near the headland where the major changes in tidal structure occur. The regular geometry of the polar model also allows the use of an idealized submarine topography and therefore changes in tidal flow due to imposed beach gradients can readily be examined. The polar model has now been developed to derive the tidal flow around an idealised circular island surrounded by a uniformly sloping beach gradient. In the absence of effects due to the Earth's rotation the derived tidal structure and the associated patterns of residual flow possess twofold symmetry. In a rotating reference frame, however, some asymmetry in these flow patterns can be detected. In this paper some numerical results showing the hourly tidal streams around an island are derived.
A hydrodynamic numerical model developed in polar coordinates derives the tidal characteristics around an island. Due to the frictional stress over the sloping topography four residual eddies are generated. The effect of the Earth's rotation deflects the tidal flow in the shallow waters near the island and marginally adjusts the residual flows. Relationships for the M2 tidal vorticity are derived for these effects which can be used to estimate the rectified vorticity distribution associated with tidal flow around islands. The corresponding bottom stress distributions, which are important in sediment transport studies, are briefly discussed.
Data on the distribution of dinoflagellates in the surface waters around the British Isles in July 1977 were analysed by correspondence analysis. Four communities of dinoflagellates were recognized, and the occurrence of each was related to various environmental parameters. The vertical stability of the water column is considered to be the most important factor in determining the relative abundance of individual species.
Whilst there have been a number of fundamental studies illustrating the effects of Earth's rotation on the flow around submerged obstacles (Hogg, 1973) and islands (Longuet-Higgins, 1969) there appear to be relatively few studies that include both non-linear advective and frictional effects (Huthnance, 1972), and few were found describing the stagnation region that develops in the lee of the flow due to the advection of vorticity generated by the frictional stress associated with a beach gradient (Pingree & Maddock, 1979). In this paper some rotational flow patterns that can develop around an island due to these effects are examined using a numerical model and an overall attempt has been made to illustrate the possible use of numerical models as a tool for theoretical investigations in areas of study that until recently have been exclusively analytical or experimental. An island is surrounded by a uniform beach gradient and the model illustrates how varying bottom topography modifies a steady flow due to both frictional and Earth's rotation effects. Only non-oscillating flows with steady wakes are considered here so that the vorticity sources and sinks can be examined as simply as possible. Stagnation regions due to frictional effects are known to be important in determining sediment deposition and the effects of the Earth's rotation may result in increasing asymmetry in sediment distributions (Johnson, Vogt & Schneider, 1971).
Seasonal cycles in the condition index of Mytilus edulis from three sites in southwest England are described. These are analysed in relation to host length, stage of gonad development and parasite burden by linear regression analysis. An effect on the condition index due to the presence of Mytilicola intestinalis can be detected only in the sublittoral mussels in those few winter months when the mean intensity of infestation is over about 25 parasites per host. In all cases studied, the magnitude of the effect due to variation in host length, stage of gonad development, seasonal cycles and environmental factors is greater than that due to parasitism.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cpath d='M200 752.362h200v300H200z' style='fill:%23fff;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3cpath d='M400 752.362h200v300H400z' style='fill:%23ff883e;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3c/svg%3e)