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Three methods which have been used to determine nutrient uptake hnetic parameters were compared using steady-state NH,-limited cultures of the chrysophyte Pseudopedinella pyriforrnis. The first 2 methods involved a multiple flask incubation where different concentrations of substrate were added to each flask. Method 1 used a variable incubation time, while the incubation time of Method 2 was short and constant. The third method, the perturbation method, involved one large addition of the substrate to one culture and hence the nutritional past history varied throughout the experiment. This method was used also with the diatoms Skeletonema costatum and Chaetoceros debilis in Si-Limited chemostats. Results inhcate that, for nutrient-limited cultures, kinetic parameters are best estimated using multiple additions of the substrate and a short constant incubation time (Method 2). It appears that this method determines membrane transport, which is still not completely free of feedback inhibition even when the incubation time is very short (e.g. 2 min). The short incubation time is necessary because the maximal uptake velocity (V,) decreases with increasing incubation time especially for phosphate and ammonium. Method 3 provides valuable information on a third parameter, V,, the approximate assimilation rate of the limiting nutrient, that is not obtained by the other methods. Multlple sequential additions of the limiting nutrient to N-or Si-limited Skeletonema costaturn and Chaetoceros debilis revealed that if the additions were small (e.g. 2 pM NH,), there was no change in subsequently determined nutrient uptake kinetic parameters. If the sequential additions were larger (e.g. 6 PM) then the maximal uptake rate slowed with time.

Different components of the food web in the Strait of Georgia are reviewed. The phytoplankton are dominated by diatoms; however, flagellates may dominate in the winter. Chlorophyll a concentrations may range from < 1 mg∙m−3 in the winter to > 15 mg∙m−3 during blooms. The average annual primary productivity is about 280 g C∙m−2 for the strait, but it is higher in frontal areas at the north and south ends of the strait and near the Fraser River plume. Light limits primary productivity during the winter months, while nutrients (nitrogen) and grazing are the limiting factors during the late spring and summer. Turbidity and salinity effects occur near the Fraser River plume. The surface macrozooplankton community is composed chiefly of copepods. Mid- and deep-water communities consist of euphausiids, chaetognaths, and some deep-living copepods, which overwinter at depth. The standing stock of macrozooplankton (> 350 μm) to 400 m, ranges from 0.1 to 2.0 g wet wt∙m−3. Few estimates of secondary production and standing stock estimates of microzooplankton have been made. Horizontal patches of zooplankton have been encountered and may be important feeding sites for some fish. Standing stock associations of the dominant species in the food web of the strait are reasonably well known, but assessment of food web dynamics from these limited standing stock measurements is often inaccurate. There is a noticeable absence of data on how rate processes affect standing stocks, and it is particularly an understanding of these interrelationships that is needed for fisheries management. There is an urgent need for more interaction between biological oceanographers and fisheries scientists, particularly in the area of zooplankton grazing by larval fish.

SUMMARY1. The location of a group of interneurones projecting to contralateral motor nuclei has been established using retrograde transneuronal transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP). After labelling the motoneurones of semitendinosus, medial gastrocnemius or quadriceps muscles, interneurones which were secondarily labelled were found in lamina VIII and in the neighbouring narrow strip of lamina VII. They were found to be distributed from the 4th lumbar to the 1st sacral segments, with the highest concentration in the 6th and 7th lumbar segments and at the border between the 4th and 5th lumbar segments.2. The electrophysiological properties of lamina VIII interneurones of the 6th lumbar segment have been investigated using both extracellular and intracellular recording. Many of these interneurones could be antidromically activated following weak stimuli applied in contralateral motor nuclei.3. Post-synaptic potentials were evoked from a variety of primary afferents including group I muscle afferents. However, when present, the post-synaptic potentials (p.s.p.s) of group I origin were of considerably smaller amplitudes than p.s.p.s. evoked from higher threshold muscle or cutaneous afferents and smaller than p.s.p.s. which followed stimulation of the spinal cord at the thoracic level. P.s.p.s. from the latter two sources appear to constitute the main input to lamina VIII interneurones.4. Group I input has been found in forty lamina VIII interneurones. These were usually affected by either ipsilateral or contralateral group I afferents and only exceptionally by both.

SUMMARY1. Intracellular recordings have been made from laminae V-VI interneurones interposed in pathways of non-reciprocal inhibition of motoneurones from group I afferents of triceps surae and/or plantaris.2. A comparison of actions of brief stretches of triceps surae and plantaris with actions of electrical stimulation of nerves of these muscles revealed that I a afferents influenced about a half of the interneurones excited by lb afferents.3. Electrical stimulation of seven muscle nerves, three cutaneous nerves, posterior knee joint and interosseous nerves, the red nucleus and the pyramidal tract, evoked excitatory post-synaptic potentials (e.p.s.p.s) in various proportions of interneurones.4. Most of the interneurones were excited monosynaptically, or both monosynaptically and disynaptically by group I afferents. This, together with a very small proportion of interneurones in which e.p.s.p.s were evoked only disynaptically, indicates that the same last-order interneurones may subserve di-and trisynaptically evoked inhibition of motoneurones. Other fibre systems excited these interneurones monosynaptically (interosseal, rubro-and corticospinal), disynaptically (cutaneous, rubro-and corticospinal) and polysynaptically. The coupling of the earliest e.p.s.p.s from group II and joint afferents could not be established, but was consistent with their mediation by only one or two interposed interneurones.5. Inhibition was evoked from all fibre systems with excitatory input to the same or to other interneurones of the sample, except from group II afferents, the effects of which were found in a much smaller number of cells and may not be fully representative.

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SUMMARY1. The technique of recording post-synaptic potentials from a population of motoneurones, by recording from ventral roots perfused with isotonic sucrose, has been applied to investigate the action of single last-order interneurones; the target motoneurones were in either caudal L7 or S1 segments.2. Using spike-triggered averaging, the inhibitory action of 70 % of previously identified last-order interneurones (Renshaw cells and lamina VII Ia inhibitory interneurones) has been detected.3. Previous observations had suggested that interneurones mediating disynaptic non-reciprocal inhibition from group I muscle afferents should be characterized by (i) location in laminae V-VI, (ii) monosynaptic group I input and (iii) ascending collateral axonal projection to upper lumbar segments. 65 % of interneurones with these characteristics were found to inhibit motoneurones. 4. In addition, spike-triggered averaging from this group of laminae V-VI interneurones sometimes revealed a depolarizing potential which preceded the inhibitory potential evoked by the interneurone. The depolarizing potential is interpreted as being due to the action of some presynaptic fibres which branch to innervate both the investigated interneurones and motoneurones.