Magnitude Of Seasonal Variation Research Articles

Spatial and temporal variability of wave energy resource in the eastern Pacific from Panama to the Drake passage

We analyse the wave energy resource available along the Pacific coast of South America from Panama from the latitude of 8°N to the Drake Passage at 55°S. The analysis is based on wave time series over 63 years (1959–2021) from the European Union Copernicus database constructed using the WAM wave model for the entire Pacific forced by wind information from ERA5. The novel features are the analysis of temporal variations in the wave energy flux, quantification of the contribution of swells and wind-seas into the wave energy potential, establishing properties of the most energy-carrying wave conditions, and evaluation of the role of El Niño and La Niña in the wave energy potential. The annual average wave energy flux increases from about 2 kW/m just to the north of the equator on the Colombian Pacific coast to 20–50 kW/m in the central and southern mainland of Chile and up to 80 kW/m near the Drake Passage. The wave energy resource to the north of latitude 32°S is almost entirely provided by swells. To the south of 44°S wind-seas predominate among the most energetic wave conditions and the maxima of energy flux by wind-seas up to 20 times exceed the already high average energy flux. The temporal variation in the wave energy flux follows the same pattern. It is fairly small at lower latitudes and increases rapidly from the forties. The magnitude of seasonal variation in terms of monthly mean wave energy flux is commonly from −47% to +32% from the long-term mean. The calmest time that contains 1% of the total annual energy flux is 10–20 days, about 10% of energy is contained in the 100 calmest days, while 50% of the annual energy flux arrives during the 100 days with strongest waves in the entire study area. The typical height of waves that provide the largest contribution to wave energy is about 1–1.5 m in the very north, around 1 m near the equator, gradually increases to the South and reaches 3.5–4 m on the shores of southern Chile. The associated wave periods are about 10 s in the entire study area. The wave energy flux has been almost constant over the 63 years near the equator but has increased at a rate up to 0.6 kW/m per year in the nearshore of Chile. For the evaluated time scales in coastal areas of South America, the interchange of El Niño and La Niña does not have detectable impact on the wave energy resource.

Structure and variations of global planetary boundary layer top from 2008-2022 multiple GNSS RO observations

The structural changes at the planetary boundary layer (PBL) top are very complex and closely related to climate and environmental changes. With the development of Global Navigation Satellite System Radio Occultation (GNSS RO), it provides a good opportunity to estimate and study PBL variations. In this paper, long-term variations and structures of planetary boundary layer height (PBLH) from 2008 to 2022 are investigated by the Wavelet Covariance Transform (WCT) method based on refractivity profiles from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Korea Multi-purpose Satellite-5 (KOMPSAT-5). Furthermore, the temperature and pressure of PBL top are detected using the temperature and pressure profiles from COSMIC and KOMPSAT-5 occultation data. The results demonstrate the latitudinal distribution of the PBL top height, temperature, and pressure, with the more apparent temperature's latitudinal characteristics. The pressure is more strongly correlated with PBL height than temperature. The PBL height in most land and oceans is high in summer and low in winter, and the PBL pressure is low in summer and high in winter, according to seasonal variation characteristics. The global PBL temperature is high in summer and low in winter, which is more obvious in high latitude region. A positive correlation between latitude and the magnitude of seasonal variation in PBL temperature is observed worldwide. Seasonal variation is less pronounced in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH) due to sea-land differences. The annual average value of the global PBL height, temperature, and pressure do not change significantly over the long term, and also there is no a discernible upward or downward trend.

Seasonal variations in daily data: An application to air passenger arrivals

The aim of this paper is to describe and compare seasonal effects in daily air passenger arrivals. Multiple seasonal cycles of different lengths are usually observed in daily time series. To model weekly cycles over a short and fixed seasonal period, conventional formulations can be applied. However, the length of yearly and, above all, monthly seasonal cycles is not fixed. Yet, whatever the lengths of the seasonal periods, seasonal cycles can be modelled by using evolving spline functions in such a way that a seasonal effect at a proportion of the seasonal period is defined as a non-fixed parametric formulation of this proportion. Thus, restricted evolving spline models are useful to model this type of seasonal variation. The areas under the splines can measure changes in the magnitude of seasonal variations over time and can be used to compare the relevance of seasonal variations for different seasonal periods or different time series. This proposal is illustrated by applying it to daily series of air passengers arriving in the Canary Islands and at the rest of Spain's airports.

Superhabitability of High-obliquity and High-eccentricity Planets

Planetary obliquity and eccentricity influence climate by shaping the spatial and temporal patterns of stellar energy incident at a planet’s surface, affecting both the annual mean climate and magnitude of seasonal variability. Previous work has demonstrated the importance of both planetary obliquity and eccentricity for climate and habitability, but most studies have not explicitly modeled the response of life to these parameters. While exaggerated seasons may be stressful to some types of life, a recent study found an increase in marine biological activity for moderately high obliquities <45° assuming an Earth-like eccentricity. However, it is unclear how life might respond to obliquities >45°, eccentricities much larger than Earth’s, or the combination of both. To address this gap, we use cGENIE-PlaSim, a 3D marine biogeochemical model coupled to an atmospheric general circulation model, to investigate the response of Earth-like marine life to a large range of obliquities (0°–90°) and eccentricities (0–0.4). We find that marine biological activity increases with both increasing obliquity and eccentricity across the parameter space we considered, including the combination of high obliquity and high eccentricity. We discuss these results in the context of remote biosignatures, and we argue that planets with high obliquity and/or eccentricity may be superhabitable worlds that are particularly favorable for exoplanet life detection.

Dynamic factor analysis of seasonal variation in daily physical activity in individuals with heart failure and implanted cardiac devices

BackgroundThe purpose of the present study was to determine the presence and magnitude of seasonal variation in daily physical activity (PA) in those with heart failure (HF). MethodsRetrospective study and dynamic factor analysis (DFA) of Patient Activity data from Medtronic implanted cardioverter defibrillator and cardiac resynchronization devices (ICD/CRTs). ResultsIn a data set of 435 patients, distinct states/trends were identified by DFA including a classic, sinusoidal pattern of seasonal variation and a pattern of decline over the course of 12 months, which were associated with specific clinical characteristics. Overall, model fitting was good. ConclusionsThose with low comorbidities, better NYHA Class, higher BMI, no hospitalization, and male sex demonstrated greater seasonal variation of at least 40 min per day between winter (lowest PA) and spring/summer (highest PA). Those with female sex and hospitalization demonstrated overall downward trajectories of approximately 40 and 80 min, respectively, over the course of the year.

Features of Seasonal Variability of Tidal Sea-level Oscillations in the Russian Arctic Seas

Tides play an important role in the mesoscale sea level variability in the Arctic seas. A distinctive feature of tides in this region is the seasonal variability of harmonic constants, which is caused by changes in hydrometeorological conditions during a year. Based on the analysis of long-term hourly sea-level observations in the White, Chukchi, and Laptev seas, the seasonal variability of the main tidal harmonic constants and some nonharmonic tidal characteristics are studied. The use of multiyear observation series allowed calculating the mean amplitudes and phases of tidal harmonics in certain months with high precision and estimating their interannual variability. Significant differences are also found in the pattern and magnitude of seasonal variability both between stations and between the White Sea and the Eastern Arctic.

1200 Seasonal Variation In RHR Measured By A Wearable Ring

Abstract Introduction Resting heart rate (RHR) associates with cardiovascular fitness, acute and chronic health status, and mental stress. Nightly sleep provides an excellent measurement time because ambient conditions are constant and confounding factors are controlled. Relatively low RHR is seen as a mark of better health, performance, and recovery levels. Over the past few years, development of wearable devices has made it possible to follow the course of individual RHR as long-term time series, which enables observation of how behavioural, societal and seasonal factors affect RHR at population scale. Methods In this study, mean individual nightly RHR of each calendar day was studied from all Oura ring users starting from July 2016 (n=1415) and ending at March 2019 (n=57.278). The subjects were 36 percent female and 64 percent male, mostly in working age: 20-30-year (15%), 30-50 years (60%), and 50-60-years (16%). Majority of the ring users live in the Northern Hemisphere, and minority (n = 1.500) in the Southern Hemisphere. Results With users from Northern Hemisphere, the yearly RHR peaks in December and lowers between April and August. The magnitude of seasonal variation was observed to be roughly 2 bpm: In 2018, March averages 1.6 bpm higher than August, and December averages 2.1 bpm higher than August. Looking at subjects from New Zealand and Australia, exactly opposite phase was observed, so that both hemispheres followed the length of the day. Apparently, RHR yearly fluctuations are based on biological factors like Sun light and ambient temperature. Both hemispheres also demonstrated weekly peaks at weekends (+1.6 bpm), and a most district peak at each New Year night. Conclusion The long-term RHR was shown to be affected by biological, societal, and lifestyle factors. We find that long-term wearable data from the Ring can reveal health related physiological effects across large populations, including seasonality. Support This work was supported by Oura Health Ltd.

Quantification of Seasonal Variation in Daily Physical Activity in Individuals with Heart Failure and Implantable Cardioverter Defibrillator/Cardiac Resynchronisation Therapy Devices.

Understanding seasonal variation of daily physical activity (PA) in patients with heart failure (HF) has important implications for planning and interpretation of clinical trials, but the presence and magnitude of this seasonal variation in daily PA have yet to be established. The purpose of the present study was to determine the presence and magnitude of seasonal variation in daily PA in community-dwelling individuals with HF using several analytic approaches. Retrospective chart review of patients with HF and Medtronic implantable cardioverter defibrillator/cardiac resynchronisation therapy (ICD/CRT) devices. Data included in analyses (autocorrelation, analysis of covariance, one-way analysis of variance) were clinical characteristics, the patient activity measure of daily PA from the ICD/CRT devices, temperature and hours of daylight over the 1-year period of 1 November 2017-31 October 2018. One hundred and sixty-eight patients were included. Visual analysis and autocorrelation demonstrated seasonal variation in daily PA. Daily PA seasonal difference between winter and summer months was 0.4 hours per day/24 minutes per day/2.8 hours per week/14.9%. This seasonal effect on daily PA is significantly greater in those with ≤8 comorbid conditions and an overall activity level of >2.2 hours per day compared to those with multiple comorbidities and low overall activity (0.7 versus 0.1 hours per day, respectively). The present study affirms the seasonality of daily PA in a cohort of patients with HF and ICD/CRT devices and reveals a disproportionate seasonal effect on those with fewer comorbidities and higher overall activity levels. Seasonal variation should be accounted for when interpreting change in daily PA in clinical practice and when designing and interpreting results of clinical trials investigating interventions to improve daily PA.

Evolution of plasticity and adaptive responses to climate change along climate gradients.

The relative contributions of phenotypic plasticity and adaptive evolution to the responses of species to recent and future climate change are poorly understood. We combine recent (1960-2010) climate and phenotypic data with microclimate, heat balance, demographic and evolutionary models to address this issue for a montane butterfly, Colias eriphyle, along an elevational gradient. Our focal phenotype, wing solar absorptivity, responds plastically to developmental (pupal) temperatures and plays a central role in thermoregulatory adaptation in adults. Here, we show that both the phenotypic and adaptive consequences of plasticity vary with elevation. Seasonal changes in weather generate seasonal variation in phenotypic selection on mean and plasticity of absorptivity, especially at lower elevations. In response to climate change in the past 60 years, our models predict evolutionary declines in mean absorptivity (but little change in plasticity) at high elevations, and evolutionary increases in plasticity (but little change in mean) at low elevation. The importance of plasticity depends on the magnitude of seasonal variation in climate relative to interannual variation. Our results suggest that selection and evolution of both trait means and plasticity can contribute to adaptive response to climate change in this system. They also illustrate how plasticity can facilitate rather than retard adaptive evolutionary responses to directional climate change in seasonal environments.

Spatial and Temporal Patterns of CO2, CH4, and N2O Fluxes at the Soil-Atmosphere Interface in a Northern Temperate Forested Watershed

Forest soils are recognized as sources and sinks of greenhouse gases (GHG) (CO2, CH4, N2O), but there are limited data quantifying the magnitude of GHG fluxes at the soil-atmosphere interface across a range of landscape hydrogeomorphic conditions. In our study, GHG fluxes were measured in a forested watershed across a range of hydrogeomorphic locations (wetlands, hillslopes, riparian zones, etc) and evaluated in relation to temperature, antecedent flow conditions, and stream chemistry to help develop strategies to scale GHG emissions from the point scale to the watershed scale. Mean study period CO2 fluxes (0.61 to 2.89 gCm-2d-1) were positive at all sites, with larger fluxes occurring in well-drained soils. Negative fluxes (CH4 sinks) were found at the hillslope and lowland sites, while the wetland was a large source of CH4 emissions at the watershed scale. Mean CH4 fluxes ranged from -2.53 to 330.34 mgCm-2d-1. Nitrous oxide fluxes were low relative to other GHG fluxes (in terms of CO2 equivalent) and ranged between -0.72 to 0.70 mgNm-2d-1. Although carbon dioxide fluxes were positively correlated to soil temperature at all locations, CH4 and N2O fluxes were not significantly related to temperature, antecedent flow conditions, or stream chemistry at the watershed scale. However, strong differences in CO2 and CH4 fluxes related to landscape geomorphology were observed, and exceeded the magnitude of seasonal variations for CO2 and CH4 fluxes, suggesting that landscape hydrogeomorphology was likely a stronger predictor of GHG fluxes at the watershed scale than temperature and stream chemistry variables, at least within the confine of one watershed. In lieu of statistical approaches relying on environmental variables to predict GHG fluxes at the watershed scale, geomorphological approaches, potentially coupled with seasonal comparisons of GHG fluxes in each land class, might therefore be a promising research avenue to provide solid watershed wide estimates of GHG fluxes.

Seasonal small-scale variation in distribution among depth zones in a coastal Baltic Sea fish assemblage

AbstractWe studied seasonal and small-scale spatial variation in fish assemblage structure in the northern Baltic Proper archipelago. The study was conducted in a shallow coastal basin during three consecutive production-seasons. The structure of the fish assemblage changes significantly seasonally, from early summer (May–June) to late summer (August–September), and spatially over short distances (ca. 500 m) and small depth intervals (ca. 5 m) in an area without physical barriers. The magnitude of the seasonal variation was depth zone-specific, indicating that seasonal patterns from a given depth zone cannot be directly extrapolated to adjacent ones, let alone to a whole water body. In early summer, the adult fish displayed spawning aggregations, and their abundance was highest closest to the shoreline. In late summer, the adult fish were more evenly distributed and the assemblage was dominated by high abundances of juvenile fish. The results underline the importance of including several spatial and temporal scales into studies on fish distribution. The resulting patterns from such studies may appear idiosyncratic unless the nature and magnitude of seasonal variation and small-scale depth zone distribution are taken into account.

Drivers of column-average CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; variability at Southern Hemispheric Total Carbon Column Observing Network sites

Abstract. We investigate factors that drive the variability in total column CO2 at the Total Carbon Column Observing Network sites in the Southern Hemisphere using fluxes tagged by process and by source region from the CarbonTracker analysed product as well as the Simple Biosphere model. We show that the terrestrial biosphere is the largest driver of variability in the Southern Hemisphere column CO2. However, it does not dominate in the same fashion as in the Northern Hemisphere. Local- and hemispheric-scale biomass burning can also play an important role, particularly at the tropical site, Darwin. The magnitude of seasonal variability in the column-average dry-air mole fraction of CO2, XCO2, is also much smaller in the Southern Hemisphere and comparable in magnitude to the annual increase. Comparison of measurements to the model simulations highlights that there is some discrepancy between the two time series, especially in the early part of the Darwin data record. We show that this mismatch is most likely due to erroneously estimated local fluxes in the Australian tropical region, which are associated with enhanced photosynthesis caused by early rainfall during the tropical monsoon season.

The role of metabolism in understanding the altitudinal segregation pattern of two potentially interacting lizards

Sympatric species from the same ecological guild, that exhibit partial altitudinal segregation, can potentially interact in areas of syntopic occurrence. Besides general species' ecology, physiology can provide important answers about species interactions reflected in altitudinal patterns. Lizards Podarcis muralis and Iberolacerta horvathi exhibit partial altitudinal segregation, while they strongly resemble in overall morphology and ecology (diet, daily and seasonal activity pattern), but show certain degree of physiological dissimilarity. They have similar mean preferred body temperatures and patterns of seasonal and daily variations but differ in the magnitude of seasonal variation. Since an ectotherm metabolism is highly dependent on body temperature, thermoregulation is expected to directly affect their metabolism. We compared metabolic rates of adult males from an area of sympatry, measured under two temperature regimes (20°C and 28°C). Both species increased metabolic rates with temperature in a similar pattern. We also compared electron transport activity from tail tissues which provide values of species' potential metabolic activity (enzymatic capacity). Species clearly differed in potential metabolic activity; I. horvathi attained higher values than P. muralis. No difference was detected in how species exploited this potential (calculated from the ratio of electron transport activity and metabolic rates). However, we observed higher potential metabolic activity I. horvathi which together with the ability to thermoregulate more precisely could represent a higher competitive advantage over P. muralis in thermally more restrictive environments such as higher altitudes. Understanding of metabolism seems to provide valuable information for understanding recent distributional patterns as well as species interactions.

Oxygen regime of rivers in Volga, Ob, and Lena basins

A close negative correlation was found to exist between oxygen concentration in winter and the latitude in rivers in the basins of the Ob and Volga. This can be explained by an increase in the duration of the freeze-up period (during which oxygen is consumed for oxidation of organic matter (OM) in water and bottom sediments) in the direction from the south to the north, while the input from oxygen from the atmosphere and photosynthesis is practically zero. No inverse relationship was found to exist between dissolved oxygen and the latitude, because in rivers of mountain and semimountain types, typical of this basin, organic matter (OM) does not accumulate in bottom sediments and the conditions of atmospheric circulation are better than in lowland rivers. The magnitude of seasonal variations in oxygen in lowland rivers is greater than in mountain ones because of an abrupt drop in oxygen concentration in slowly flowing rivers in winter and the saturation of their water by oxygen from photosynthesis in summer.

Environmental zonation across the Australian arid region based on long-term vegetation dynamics

Zonation of landscapes is generally based on broad scale biophysical data, field surveys, imagery and expert knowledge. Such zonation represents a static view of the environment and does not reflect dynamics and function. Arid environments are however often highly dynamic, and spatial and temporal patterns may be expressed over long periods of time. These dynamics need to be understood for management. Our aim is to understand the dynamics and functional response of vegetation in the Australian arid zone, and use this to inform and potentially improve the currently employed stratification. Principal component analysis of 25 years of satellite imagery identified underlying factors influencing patterns of arid vegetation growth, and regions of similar long-term response. Dominant factors of variation were identified as the spatial distribution of total vegetation growth, seasonality of growth, magnitude of seasonal variability in growth, and regularity of variation in growth. Additional variation resulted from episodic vegetation growth of limited spatial extent and duration. Classes expressing these functional components were compared with the existing biogeographical regions, revealing agreement in some instances, and in other cases adding information previously not available. The study demonstrates a new approach to Australian landscape zonation that has potential for much wider application.

Platypus (Ornithorhynchus anatinus) body size, condition and population structure in Tasmanian river catchments: variability and potential mucormycosis impacts

Context Despite widespread interest in platypus (Ornithorhynchus anatinus) conservation, it is unclear how their fundamental morphometric and demographic characteristics differ over a range of scales. This hampers impact assessments and understanding of platypus ecology. Although the ulcerative fungal disease mucormycosis has infected platypuses in Tasmania for three decades, its population level impacts and conservation significance remain unknown. Aims This study examined morphometric and demographic patterns in Tasmanian platypuses to provide a basis for investigating impacts of mucormycosis or other anthropogenic disturbances. It also sought to identify important spatial scales of natural variability and the magnitude of seasonal variation in platypus body size, condition and population structure. The hypothesis of higher mucormycosis prevalence and mortality in adult males was also investigated. Methods Extensive live-trapping surveys were conducted from January 2008 to July 2009 in 75 streams and 18 river catchments across Tasmania including King Island. The sex, age, body size, tail volume index, health and moult condition of 195 individuals were assessed, and population age and sex structures characterised. Sampling focussed on assessing variability within and between river catchments and compared populations in river catchments with contrasting disease status. Key results Differences in platypus morphometrics within and between catchments and seasonal moulting patterns were detected. Adult males had higher fat stores than adult females, especially during winter. This study also provided the first evidence of population level consequences of disease in platypuses. The demographic group most commonly affected by mucormycosis was confirmed to be adult males. Differences in male age structure among catchments of varying disease status were consistent with the hypothesis of higher adult male mortality rates and turnover in currently affected catchments. Conclusions More than 25 years after mucormycosis was first detected in Tasmanian platypuses, the disease continues to play a low-level, ongoing role in affected populations. Implications The present study provides the first systematic multi-scale spatial investigation of platypus mucormycosis, which contributes to unravelling the epidemiology of the disease and detecting its impacts. By identifying the magnitude and important scales of morphometric and demographic differences in Tasmanian platypuses the study also assists researchers choose comparable demographic groups and spatial scales for meaningful comparisons in future impact studies.

Decadal changes in surface carbon dioxide and related variables in the Mediterranean Sea as inferred from a coupled data-diagnostic model approach

Abstract Louanchi, F., Boudjakdji, M, and Nacef, L. 2009. Decadal changes in surface carbon dioxide and related variables in the Mediterranean Sea as inferred from a coupled data-diagnostic model approach. – ICES Journal of Marine Science, 66: 1538–1546. A coupled approach based on available datasets of temperature, salinity, oxygen, nutrients, and chlorophyll, and a surface layer box model previously developed and modified for the present study, allowed us to reconstruct dissolved inorganic carbon (DIC), total alkalinity, and carbon dioxide fugacity (fCO2) mixed-layer fields for the Mediterranean Sea, from the 1960s to the 1990s. The approach used in this study resulted in a 7% relative error on reconstructed surface fCO2 fields. The Mediterranean Sea transformed from a source of 0.62 Tg C year−1 for atmospheric CO2 in the 1960s, to a net sink of −1.98 Tg C year−1 in the 1990s. The annual cycle in surface fCO2 was driven mainly by temperature variations in the Mediterranean Sea, whereas its decadal variations resulted from a balance between primary production and the thermal effect. According to our model results, the atmospheric CO2 increase of ∼40 µatm over the period of our investigation induced an increase in DIC of ∼30 µmol l−1 in surface waters. A 50% reduction in the magnitude of seasonal variations in surface temperature occurred during the 1990s relative to the earlier decades. Therefore, surface fCO2 only increased by 24 µatm from the 1960s to the 1990s. Changes in pH were not significant over this period.

Ecophysiology in environmental impact assessment: implications of spatial differences in seasonal variability of bivalve condition

Nucleic acid ratios (RNA:DNA, RNA:protein or total RNA content) have been success- fully used as indirect measures of short-term growth in a range of marine organisms. However, using biochemical and physiological indicators in environmental impact assessment requires changes to be evaluated against a background of natural variability (e.g. spatial differences in the seasonal variabil- ity and size-dependence in bivalve condition). The natural temporal and spatial variability in nucleic acid ratios and condition indices in adult cockles Austrovenus stutchburyi was investigated along a gradient of increasing sediment grain size and wave exposure in an estuary on the east coast of New Zealand. The results were examined in relation to bivalve reproductive state, water temperature, and sediment and hydrodynamic characteristics of the estuary. The magnitude of seasonal variability in RNA was site-specific and increased towards more exposed sites, likely due to enhanced food avail- ability at these sites. In addition, investigation of the effect of bivalve size class on nucleic acid ratios and condition indices revealed site-specific patterns. Thus, by integrating information over a range of spatial and temporal scales, we can increase our understanding of the interaction between the phys- iology of estuarine soft-sediment bivalves and their spatially and temporally complex environment to provide a sound basis for assessing the effects of added environmental stressors.

The Variability of Seasonality

AbstractSeasons are the complex nonlinear response of the physical climate system to regular annual solar forcing. There is no a priori reason why they should remain fixed/invariant from year to year, as is often assumed in climate studies when extracting the seasonal component. The widely used econometric variant of Census Method II Seasonal Adjustment Program (X-11), which allows for year-to-year variations in seasonal shape, is shown here to have some advantages for diagnosing climate variability. The X-11 procedure is applied to the monthly mean Niño-3.4 sea surface temperature (SST) index and global gridded NCEP–NCAR reanalyses of 2-m surface air temperature. The resulting seasonal component shows statistically significant interannual variations over many parts of the globe. By taking these variations in seasonality into account, it is shown that one can define less ambiguous ENSO indices. Furthermore, using the X-11 seasonal adjustment approach, it is shown that the three cold ENSO episodes after 1998 are due to an increase in amplitude of seasonality rather than being three distinct La Niña events. Globally, variations in the seasonal component represent a substantial fraction of the year-to-year variability in monthly mean temperatures. In addition, strong teleconnections can be discerned between the magnitude of seasonal variations across the globe. It might be possible to exploit such relationships to improve the skill of seasonal climate forecasts.

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.