Magnitude Of Seasonal Variation Research Articles

Contrasting Relationships Between Nutrients, Chlorophyll a and Secchi Transparency in Two Shallow Subtropical Lakes: Lakes Okeechobee and Apopka (Florida, USA)

ABSTRACT Ten years of monthly water quality data were compared from two large shallow lakes in Florida, USA – Lakes Okeechobee and Apopka. Seasonal changes in trophic state index (TSI) values and log-log regression models relating total phosphorus (TP), total nitrogen (TN), chlorophyll a (CHLA), and Secchi transparency (SD) were considered. The objective was to quantify the extent to which empirical models might vary due to the heterogeneous nature of benthic-pelagic coupling that can occur in shallow lakes. In the offshore region of Lake Okeechobee, TP and SD-based TSI values increase dramatically during winter, while CHLA-based TSI declines. These changes coincide with the windy season in south Florida, when average wind velocities can exceed 20 km h−1. Resuspension of bottom sediments occurs and this reduces light penetration to the extent that the growth of phytoplankton is inhibited. During summer, winds are calm, and these conditions occur less often. Log-log regression models of CHLA vs. TP have a negative slope during winter and a positive slope during summer in offshore Lake Okeechobee. The slope of SD vs. CHLA models displays the opposite seasonal pattern. Seasonal changes are considerably muted in the near-shore region, where sediments are more consolidated and where shallower depths reduce the severity of light suppression of phytoplankton growth. The magnitude of seasonal variation in TSI values in Lake Apopka is muted, perhaps because seasonal variation in wind velocity is less than in the region of Lake Okeechobee. Hypereutrophic levels of nutrients, CHLA, and SD occur year-round. The log-log regression models of CHLA vs. TP and CHLA vs. TN are highly significant, and light availability does not appear to limit rates of algal production. Frequent resuspension events may occur, but negative effects on phytoplankton may be mitigated by: (1) shallower depth; (2) a greater presence of viable algae in the surface sediments that might “seed” the water column with phytoplankton; (3) high concentrations of soluble nutrients in sediments that might stimulate algal growth; and (4) lower concentration of light-attenuating inorganic particles than in Lake Okeechobee. The results indicate that a high degree of variability in both the seasonal patterns and relationships among nutrients, CHLA, and transparency can occur in shallow subtropical lakes, even when they share common features (e.g., geographic location, large size, wind patterns). This finding has practical implications regarding the use of regression models in the context of shallow lake management. Models may be quite useful on a lake-by-lake basis (e.g., in Lake Apopka they have successfully been used in eutrophication management), but considerable care should be taken when generalizing models to other lake ecosystems.

Seasonal changes in condition, nutrition, gonad maturation and energy content in barbel, Barbus sclateri, inhabiting a fluctuating river

The objective of this study was to investigate the seasonal cycle of condition, nutrition and gonad development, as well as the magnitude of seasonal variations in energy content of somatic and gonad tissues in juveniles, males and females of Sclater's barbel, Barbus sclateri, from the upper Guadalete River (south Spain). The influence of reproductive cycle on somatic changes was also investigated and discussed. Measurements of condition factor (K), somatic index (SI), gonadosomatic index (GSI) and somatic and gonad energy content (J g-1 dw) were made in individual specimens taken from the Guadalete River monthly for 12 months. This freshwater ecosystem is characterised by strong seasonal fluctuations in water and flow level, temperature and food supply. It was found that in general juveniles, males and females of barbel exhibited a similar condition, nutrition and somatic energy cycle throughout the year, with the highest values in spring and the lowest in summer. Depletion of K, SI and somatic energy storage from spring until mid-summer seems to be associated with high metabolic demands during this period, and in adult fish also with spawning-related activity. The gonad index and energy content of the gonad were the highest in April and the lowest in summer for both sexes. Spawning took place during late spring – early summer, with fish quiescent by mid summer. The energy required for ovarian development (3970 J g-1 dw) was greater than the one for testes development (2763 J g-1 dw). Data on gonad energy content indicated a period (March to April) of intense energy accumulation (64% males and 37% females) which was related to the decline in the average somatic energy content in males and females. The somatic energy content was linearly related to K and SI. In the same way, GSI correlated positively with gonad energy. Linear trends were found between somatic parameters (K, SI and energy content) and gonad parameters (GSI and energy content).

Circannual rhythm of measles subsequent to immunisation.

This is an epidemiological study of biennial and annual rhythms of measles in England and Wales between 1959-1994 highlighting changes following immunisation. The study describes annual rhythms starting in autumn (Week 41) and the statistical method tests the fit of a sine curve to annual data. Before immunisation the previously established biennial rhythm was seen: the two years differed in magnitude of seasonal variation, position of the peak, "visibility of school holidays" and peak breadth. After immunisation (1968) these biennial features tended to disappear, one annual rhythm becoming the predominant feature. By the late eighties adjacent years had very similar sine curve fits. About 1990 (89-91) the fit of the sine curve was no longer significant, was of low amplitude and numbers were at an all time low. In 1992 significant annual variation returned, numbers rose and magnitude of seasonal variation increased. At this time a new epidemic was being predicted on other evidence and this renewed sine curve fit may be an additional warning signal. A possible influence of normal birth rhythm on numbers of susceptibles is described.

Seasonal variations in acid‐neutralizing capacity in 13 northeast United States headwater streams

Variations in acid‐neutralizing capacity (ANC) in 13 streams in the Adirondack, Catskill, and Northern Appalachian Plateau regions of the northeast United States were related to discharge, time of year, and seasonal variations in cation and anion concentrations using periodic regression analysis, ANC varied significantly with both discharge and time of year in 12 streams. Generation of ANC seasonal variations, being dependent upon the precise timing and magnitude of seasonal variations in cation and anion concentrations, was unique to each stream. Greatest seasonal ANC variations occurred in streams where seasonal variations in major anion and cation concentrations were completely out of phase. Maximum errors that could occur because of extrapolation of ANC data from one time of year to another were equal to or greater than maximum errors due to extrapolation of ANC from one discharge to another.

Air pressure fluctuations in the North Atlantic and their relationship with El Niño-southern oscillations

Seasonal and interannual variability of the pressure field and indices of the North Atlantic atmosphere zonal circulation are analysed using historical (1894–1988) observations. It is shown that fluctuations of the index of North Atlantic oscillations (NAO) and that of the eastward transport give evidence of the interannual fluctuations with the typical time scale being 2–7 years. It is shown that the magnitude of interannual NAO index variability exceeds the typical magnitude of seasonal variations, particularly in winter. The time scale of NAO index variations and eastward transport coincides with the typicalEl Nino-southern oscillations (ENSO) temporal scale. The amplitudes of the annual, semi-annual harmonics, and high-frequency fluctuations of the NAO index increase during a typical ENSO event at least by a factor of 2.

Seasonal variation in testis size, gonadotrophin secretion and pituitary responsiveness to GnRH in rams of two breeds differing in time of onset of the breeding season

A study was conducted to compare the variation in testis size, gonadotrophin secretion and pituitary responsiveness to an exogenous gonadotrophin-releasing hormone (GnRh) challenge during the transitional period from the non-breeding to the breeding season in rams of the Romney and Poll Dorset breeds. Testis size of rams of both breeds varied significantly ( P < 0.001) during the trial period, with a significant ( P < 0.01) breed difference in the timing and magnitude of this variation. In Poll Dorset rams, increases in test's size occurred earlier, but the magnitude of seasonal variation was smaller. Overall, mean luteinizing hormone (LH) concentration was significantly ( P < 0.05) higher in Romney rams mainly because of a difference ( P < 0.001) in the frequency of LH pulses. There was also a significant ( P < 0.01) effect of sampling time and significant ( P < 0.05) breed × time interactions in LH pulse frequency. Mean follicle-stimulating hormone (FSH) levels exhibited significant ( P < 0.01) variation with sampling time, with the increase in FSH levels occurring earlier ( P < 0.06) in Poll Dorset rams. There was a significant effect of sampling time on both the peak ( P < 0.01) and the total ( P < 0.05) LH response to GnRH challenge, but no significant effects of breed or breed × time interactions were detected. These results show that breed differences in seasonally are associated with differences in the pattern of variation in testis size and gonadotrophin secretion during the transitional period from the non-breeding to the breeding season. Some of these differences could potentially be used to select rams within a breed for date of onset of the breeding season in their daughters.

Seasonal changes in the extrathyroidal distribution of iodide in the teleost fish the burbot, Lota lota L.

The distribution of radioactivity in tissues of adult burbot weighing 1 to 3 kg was determined by measuring the T/S ratio (tissue radioactivity/serum radioactivity) at intervals after the injection of radioiodide. In one instance the skin which comprises 12% of body weight of the burbot showed a T/S ratio of 29.3 and accounted for up to 27% of the injected dose of radioiodide. By comparison, in another study, the thyroid showed a T/S ratio of 5.2, representing 2.2% of the injected dose at 9.5C. The radioactivity (T/S ratio) in the skin increased until 30 h after injection and then declined.Non-radioactive iodide values of skin and serum were determined on a natural population over the course of a year. Values for skin iodide ranged from 227 μg/100 g to 816 μg/100 g while serum iodide values ranged from 7 μg/100 ml to 30 μg/100 ml. These values also indicate the magnitude of seasonal variation, which, in the case of serum iodide concentration, parallels the seasonal changes in lake temperature.