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From February to June species of Foraminifera were the only food found in gizzards of young Retusa obtusa (Montagu) in the Forth Estuary, Scotland. Then, in July, newly settled Hydrobia ulvae (Pennant) also began to be eaten although foraminiferans continued to be consumed in increasing frequency until September. Foraminiferans became scarce in R. obtusa from October to January during which time H. ulvae increasingly dominated the diet. Foraminiferans and H. ulvae both reached their highest frequencies in the gizzards of adults during February-April. Finally, foraminiferans became predominant again as H. ulvae became rare in older, post-reproductive R. obtusa in April and May prior to the death of predators in May and June. The highest count was 31 foraminiferans in one gizzard, the overall mean was 5·6 per gizzard and monthly means ranged from 1·6 in January to 9·7 in February 1993. Throughout most of the period of predation on H. ulvae (August-January), counts of foraminiferans were markedly higher in those gizzards which lacked H. ulvae than where H. ulvae was also present. Only earlier (July) and later (February and March), did counts with H. ulvae present approach and even exceed (in March) those where H. ulvae was absent. Foraminiferans in the gizzards reflected the mudflat population in species composition [almost all Haynesina germanica (Ehrenburg)]. Yet the snails, even the biggest ones, largely restricted their diet to the smaller foraminiferans (mostly 100-150 μm diameter), rarely taking the abundant individuals measuring 200-350 μm. Foraminiferans were cleared from the gizzard in ∼12 h, implying maximum consumption in the field of ∼4800 Foraminifera m -2 day -1 in September, and a total of ∼2747 by a single R. obtusa in the course of a year's growth from February to February, after which the biggest specimens of R. obtusa soon die. It is estimated that foraminiferans supply ∼60% more food than do H. ulvae during a lifetimes's growth but that H. ulvae become most important during late growth and reproduction.

The effect of cold air advection on the estimates of daily thermal time with a base temperature of 8°C (DTT8), calculated using maximum and minimum screen temperatures (max./min. values), was examined. Screen temperature data for at least three maize-growing seasons (October—April) at Cedara, Dundee and Potchefstroom were obtained from thermohygrograph charts. Two-hourly values on the charts were used to calculate unbiased estimates of DTT8. The max./min. and two-hourly estimates of DTT8 were closely correlated. Mean bias errors (MBE) associated with the max./min. DTT8, which were defined as being equal to the difference between mean max./min. DTT8 and mean two-hourly DTT8, were positive for all seasons at the three sites. MBE in max./min. DTT8 was positively related to the fraction of days experiencing cold air advection (FDCAA), with r 2 = 0.64. Since cold air advection in South Africa is most often due to the movement inland of cold marine air, proximity to the sea may be a strong indicator of MBE in m...

Retusa aobtusa (Montagu) lives little > 1 yr with most young appearing in Feb–May, followed by death of adults in May–Jun. In the Forth estuary, the main cohort of young feed on foraminifera (≈ 200 μm) until Jul–Aug when they switch to newly settled young Hydrobia ulvae (starting at ≈ 300 μm). Most breeding occurs in Jan–Mar. By May, early stages of spermatogenesis occurred in gonads of R. obtusa as small as 1.1 mm. Fully developed spermatozoa were found in gonads of most snails > 2.0 mm, and these were stored in seminal receptacles in snails > 2.5 mm. By Sep, even snails of 1.9–2 mm had mature spermatozoa in gonads and seminal receptacles, long before any substantial breeding occurred. At no time of year did snails 85 μm diameter. Fully grown oocytes (170–330 μm diameter) first occurred in gonads of snails > 2.7 mm in Aug, again long before any substantial breeding. From Dec, gonads of snails > 2.7 mm become progressively dominated by such full-grown oocytes rather than by spermatogenic tissue. In the laboratory, R. obtusa of 3.0–5.2 mm laid very few eggs between late Jul and early Dec, reaching a mean ≈ 0.3 eggs·d −1 only in mid-Nov – early Dec 1990. Subsequently snails of similar sizes released markedly more eggs: ≈ 1·d −1 during mid-Dec 1990 – mid-Jan 1991 and, briefly, 4–5.5·d −1 in Feb and Mar. Larger R. obtusa died off in May. Manipulation of laboratory water temperature, daylength and tidal regime over periods of up to 42 d failed to show significant effects on spawning rates except that some unnatural regimes (e.g., 15°C) adversely affected survival and spawning. Although R. obtusa of 3–4.5 mm bore fully developed oocytes and spermatozoa in autumn, lowered temperature and shortened daylength did not bring forward the faster spawning characteristic of Dec–Mar. It is possible that breeding activity is not directly timed by familiar environmental signals but instead simply follows a requisite period of growth and accumulation of large oocytes in an annual cycle imposed by the provision of young Hydrobia as prey to those predators at the right size to swallow them when they first appear in Jul–Aug.

Eurytemora affinis (Poppe) fed on cultured Thalassiosira weissflogii (Grunnow) at rates of 200–34000 cells copepod−1 h−1. Feeding was delayed and diminished in bright light. In dim light, feeding was initially faster in 15‰ (27000–34000 copepod−1 h−1) than in 10‰ (23000–25000 copepod−1 h−1) and much faster than in 3‰ (6000 copepod−1 h−1). After 1–3 h, feeding continued more steadily in 3‰ (1200–6500 copepod−1 h−1) but slowed drastically in 10 and 15‰ to 200–5000 copepod−1 h−1). These patterns were maintained when copepods were first acclimated briefly to the test salinities. E. affinis fed at slightly higher rates on sterile latex beads of similar size to T. weissfloggi, fastest in 10‰ and slowest in 3‰. While the beads appeared in the guts, they did not appear in the faecal pellets and after 1 h (10, 15‰) or 3 h (3‰), their numbers in suspension recovered close to original counts. In contrast, beads infected with a marine bacterium were similarly eaten (at slightly higher rates than the sterile beads), and appeared in the guts and then in the faecal pellets, while numbers in suspension continued to fall or remained low. The contrasts between initial rapid feeding in 10–15‰ and slower steadier feeding in 3‰, and between regurgitation of swallowed sterile beads and passage through the gut of bacterially-contaminated beads, have significance for the biology of a copepod living in the upper reaches of an estuary.

Young Retusa obtusa (Montagu) in the field started eating significant numbers of newly hatched Hydrobia ulvae (Pennant) in August at sizes much smaller than might physically have been ingested. Ingested H. ulvae approached and some exceeded mean maximum edible sizes in October. Then, as R. obtusa grew to 4–5 mm and bred in December–March, prey in the gizzards again decreased in size (to x values of 0.7–0.8 mm) and fell markedly short of maximum possible sizes. This winter reduction in ingested prey size is inconsistent with the growth of most small H. ulvae , their consequent decline in abundance and the increasing preponderance of larger ones that might have been readily ingestible by the full-grown predators. In the laboratory, individual R. obtusa with equal numbers of prey sizes all ate more small (0.6–1.0 mm) than large (1.4–2.2 mm) prey over 38–49-day observation. Larger R. obtusa ate more large prey and in late November–early December the largest predators together briefly ate slightly more large than small H. ulvae before returning to a predominance of small prey. When small prey comprised only 23% of those offered, they accounted for 41% numerically of those eaten. Yet, consumption of large H. ulvae contributed heavily to estimated total AFDW intake even when few large prey were taken and especially in larger R. obtusa . R. obtusa cleared small H. ulvae from the gizzard faster than large prey: large predators cleared prey faster than small predators. Thus at 10–12 °C, 1.7–1.9-mm prey in 3.1–3.3-mm R. obtusa took 6–7 days to clear the gizzard while 0.8–1-mm prey in 5-mm R. obtusa took only 1.75–2 days. Simultaneous ingestion of two H. ulvae prolonged clearance to nearly double the time for single prey. AFDW intake, based on gizzard clearance times for single prey, was fastest (≈ 2 μg AFDW · h −1 ) when big R. obtusa ate big prey but was only 0.3 μg · h −1 when small predators ate the smallest prey. The general lack of preference for larger prey and the marked preference for small prey in full-grown breeding R. obtusa of December–March lead, in nature, to rapid intake of multiple small prey: large R. obtusa in the field contained means of 3.47 and an individual maximum of 11 small H. ulvae in February 1987 compared with the usual single H. ulvae per gizzard at most times. Prey AFDW increases steadily with shell weight and thinner shells of several small prey might be easier to dissolve in the gizzard of a big predator than the thick shell of one equivalent large prey.

In the Forth Estuary, Scotland, parasitic rhizocephalan barnacle Sacculina carcini externae or scars occurred on 64% of Carcinus maenas from the subtidal channel of the middle estuary. These became more frequent downstream, infecting 46·9% in the lower estuary basin. Male and female crabs were infected equally, and infection rates in low-tide samples did not differ from high-tide. The parasites occurred most frequently on crabs of 40–50 mm carapace width (CW) rather than the modal crab size class of 50–60 mm. Downstream, proportionately more smaller than larger crabs bore parasites whereas, further upstream, more larger than smaller crabs bore parasites. There is evidence for the annual appearance of new externae in July-October, especially in the lower estuary basin, followed by their growth and loss after about a year.

Umbonium vestiarium (L.) was found to dominate intertidal sand with a mean 11 808 m −2 (95·4 g tdw m −2 ), excluding virtually all other invertebrate fauna. Whilst spawning occurs from mid-March to August, substantial recruitment seems limited to March–May, first on the lowest levels, and then upshore as the young grow to 5–6 mm by June and as the growing young move upshore. The number of ovarian eggs varied in broad accordance with total tissue dry weight: both numbers of ovarian eggs and total dry weight in females and males increased in periods between bouts of spawning but fell during substantial spawning which was mostly close to neap tides. The relationships between planktonic-egg counts, ovarian-egg counts and tissue dry weight, together with direct weighings of ovarian eggs, suggest (a) mature eggs weigh about 2·5 μg each (dry weight), (b) 10-mm females each produced about 8800 eggs (22 mg) in the 79-day observation and probably 17 000–19 000 eggs (44 g) in the total spawning period (March–August), (c) the population as a whole produced about 30 × 10 6 eggs m −2 (75 g m −2 ) in the 79 days and probably around 60 × 10 6 eggs m −2 (150 g m −2 ) plus a similar quantity of semen in March–August, giving a total reproductive output of aproximately 300 g m −2 year −1 . As Umbonium lives for only about one year with apparently one annual major period of spawning and recruitment, it is proposed that high reproductive output, large eggs and reduced larval dispersal may be adaptively related to the short lifespan on comparatively unstable and isolated habitats. These features are likely to have significant effects upon the genetic heterogeneity of Umbonium populations across its great Indo-West Pacific range.

Severe lodging in maize lands on Cedara on the 24 February 1985 was caused by a rainstorm registering Force 10 on the Beaufort scale. Large differences in lodging existed between plots in three tillage trials. In two identical primary tillage trials, four different systems of soil preparation were being compared; in the other, direct drilling was being compared to ripping with a specialized slant-legged plough. In all three cases, standability under direct drilling was more than double that under any other treatment. It appears that the firm soil condition occurring at the surface under direct drilling provides better anchorage for the maize roots, thus preventing severe root lodging. Stem lodging constituted a greater proportion of total lodging under direct drilling than under the other tillage treatments. S. Afr. J. Plant Soil 1986, 3: 165–170

Substantial spawning of Umbonium eggs occurred on 31 of 72 consecutive days observation of Malaysian littoral plankton. All spawning occurred during daylight, with egg counts reaching maxima between 1230 and 1600 on 81% of the spawning days. Highest counts on all spawning days occurred between (a) 35 min before and (b) 4.72 h after the time of daylight high tides (mean, 2.13 h after HT). In six observed 2-week tide cycles, 74.2% of the spawning days fell among the 48.6% of total days between (a) weakest neap tides and (b) 6 days after weakest neap tides: in particular, eight of the nine especially heavy spawnings occurred 0–3 days (mean 1.1 days) after weakest neap tides. Despite shifts in the daytime timing of (a) high water and (b) weakest neap tides through successive 2-wk cycles, most spawning was into high or falling water when neap high tides occurred during the middle of the day. Eggs of ≈ 160 μm diameter developed to swimming trochophores in 3–6 h, then to veligers in 6–8 h which settled ≈ 36–48 h after spawning. It is proposed that neap spawning and rapid settlement curtail dispersal and that this has adaptive significance for Umbonium which (a) dominates widely scattered and unusual habitats, (b) lives little more than 1 yr with one main annual period of recruitment and where, therefore, (c) resettlement on the shore of origin will maintain populations without bringing juveniles into competition with parents.

Early maize development is generally less rapid in the cooler soils of tillage systems which maintain large amounts of crop residues (stubble) on the soil surface. However, soil water storage tends to increase with increasing amount of surface cover, and therefore, a study was conducted in which uniformly distributed maize stubble (92% cover) was raked away from the planter rows to produce 0, 80 and 320-mm bare soil bands (BSB) over the seed-rows, or ploughed under in accordance with standard fallow period field management practices (conventional tillage). Maximum and hourly mean soil temperatures measured in the seedzone (50-mm depth) during the planting to tassel initiation period increased with increase in width of BSB and approached the values measured under the bare surface of conventional tillage (CT). However, soil water contents decreased with increasing BSB and played the dominant role in reducing the number of days from planting to 80% maize emergence, with fewer days being required as BSB decre...

Oxygen consumption in sea water increased with size and tissue weight of Natica maculosa Lamarck at ambient temperatures (28–29.5 dgC) and salinities (31–32‰) in a fashion comparable with other naticids. When measured in submerged shore sand, oxygen consumption resembled that in sea water among smaller and some larger Natica but was much lower in several larger snails. Even using the higher measurements, respiration accounted for a remarkably low proportion of the energy consumed in Umbonium prey, and this also matches previous findings with Polinices. Limited observations indicate that production of egg collars may account for > 30‰ of the consumption in adult females. Estimation of faeces and mucus were, however, unsuccessful. It is suggested that respiration in nature might be higher than recorded hitherto because of high metabolic costs and very long duration of drilling the shelled prey.

From July 1981 to July 1982 Umbonium vestiarium (L.) on a north Penang sand shore numbered 573–11 077 m −2 (mean 4126 or 53·1 g dry tissue) near MLWN and 2164–12 414 m −2 (mean 6500 or 59·8 g dry tissue) further downshore. Heavy recruitment of young became evident in June and July 1982 and a closely corresponding cohort of young was present in July 1981. Progression of this cohort indicated that young settling in May–June grew to full size (11–13 mm diameter) by January–March the following year and that virtually all died during their second year, presumably having spawned in March–May. Recruitment of young was chiefly on the lower shore but adults came to be more abundant and predominant on the upper shore. There is some evidence of migration upshore during growth. Production is estimated at 105·4 g dry tissue m −2 y −1 (2118 kj) at the lower shore level and this is almost double the 58·8 g m −2 y −1 (1176·6 kj) at the higher level. These values represent almost the entire secondary production across much of the sand flats. Possible causes of such a marked annual cycle in the very weakly seasonal tropics of the Malacca Strait are considered and it is suggested that monsoonal changes in wind, wave action and salinity might be involved.

Eurytemora affinis (Poppe) was the dominant copepod in summer and winter surveys of the littoral zooplankton of the upper Forth estuary, Scotland. Maximum numbers were observed in the 0–10%. segment, reaching 500000· m −3 in summer and consistently exceeding main channel abundances reported from other studies. In winter, E. affinis was less abundant, and shared dominance with the rotifer Synchaeta , which occasionally reached numbers of 311000·m −3 . Tidal movements and seasonal changes in freshwater flow were both observed to effect longitudinal displacement of plankton populations. Laboratory tests of salinity-temperature tolerance in Eurytemora showed optimal conditions to be 3–10%. at low temperatures, broadly matching distribution patterns in the estuary. Salinity acclimation tests and haemolymph concentration measurements showed that copepods could adapt to changing conditions within 12 h (i.e. the duration of a tidal cycle). Acclimation extended tolerance limits in the direction of the acclimation treatment, with survival being enhanced by gradual rather than abrupt changes in salinity. Investigation of haemolymph Δ°C in a range of test salinities revealed a pattern of hyper/hypo-osmoregulation, with concentrations hyperosmotic to the external medium below 15%. and evidence of hypo-osmoregulation at salinities >20%.. Minimum haemolymph concentrations were found to be equivalent to ≈6%. sea water (Δ°C = 0.4) when held in fresh water. No evidence of salinity-associated respiratory distress was found in respiration experiments. The oxygen consumption values determined (5–7.5 μl O 2 ·mg dry wt −1 ·h −1 ) at optimal salinities and temperatures were similar to reported values for copepods of comparable size.

An investigation of water conservation as affected by different primary tillage methods was conducted on a soil of the Doveton series (fine-loamy, mixed, thermic family of Rhodic Paleudults), where a direct sampling technique was used to monitor the soil-water profiles. Soil samples were taken for the period just prior to early winter tillage to two weeks before the maize crop flowered. The results showed that tillage practices which maintain higher levels of surface residues retained more water than practices which by their mode of soil disturbance leave soil surfaces relatively ‘clean’. Measured differences in soil physical properties and possibly also surface residue distribution resulting from tillage were used to explain why reduced tillage systems provided better water conservation. S. Mr. J. Plant Soil 1985, 2: 21–26