A method for deriving net primary productivity and component respiratory fluxes from tower-based eddy covariance data: a case study using a 17-year data record from a Douglas-fir chronosequence

Conventional gap‐filling procedures for eddy covariance (EC) data are limited to calculating ecosystem respiration (R_E) and gross ecosystem productivity (P_G) as well as missing values of net ecosystem productivity (F_NEP). We develop additional postprocessing steps that estimate net primary productivity (P_N), autotrophic (R_a), and heterotrophic respiration (R_h). This is based on conservation of mass of carbon (C), Monte Carlo (MC) simulation, and three ratios: C use efficiency (CUE, P_N to P_G), R_a to R_E, and F_NEP to R_E. This procedure, along with the estimation of F_NEP, R_E, and P_G, was applied to a Douglas‐fir dominated chronosequence on Vancouver Island, British Columbia, Canada. The EC data set consists of 17 site years from three sites: initiation (HDF00), pole/sapling (HDF88), and near mature (DF49), with stand ages from 1 to 56 years. Analysis focuses on annual C flux totals and C balance ratios as a function of stand age, assuming a rotation age of 56 years. All six C balance terms generally increased with stand age. Average annual P_N by stand was 213, 750, and 1261 g C m⁻² yr⁻¹ for HDF00, HDF88, and DF49, respectively. The canopy compensation point, the year when the chronosequence switched from a source to a sink of C, occurred at stand age ca. 20 years. HDF00 and HDF88 were strong and moderate sources (F_NEP=−581 and −138 g C m⁻² yr⁻¹), respectively, while DF49 was a moderate sink (F_NEP=294 g C m⁻² yr⁻¹) for C. Differences between sites were greater than interannual variation (IAV) within sites and highlighted the importance of age‐related effects in C cycling. The validity of the approach is discussed using a sensitivity analysis, a comparison with growth and yield estimates from the same chronosequence, and an intercomparison with other chronosequences.     

Natural variation in macroinvertebrate assemblages and the development of a biological banding system for interpreting bioassessment data—a preliminary evaluation using data from upland sites in the south-western Cape,South Africa

The variability of macroinvertebrate assemblages was investigated at 27 upland reference sites in the south-western Cape, South Africa. Multivariate analyses showed that sites did not group on the basis of geomorphological zonation, i.e. mountain stream and foothill-cobble bed. When separate analyses were undertaken for mountain stream (n = 21) and foothill-cobble bed sites (n = 6), assemblages formed three and two groups, respectively. Similarity amongst groups ranged from 47% to 52%, while within-group similarity was between 54% and 67%. Environmental variables shown to contribute to this variability included distance from source, cation ratio ([Na⁺]+[K⁺]/([Na⁺]+[K⁺]+[Ca²⁺]+[Mg²⁺]), pH, longitude and stream width. Whilst overall variability in the metrics of the biotic index, SASS (South African Scoring System), is high at reference sites, the interpretation of monitoring-site data using biological bands derived from a range of reference sites, ensured that variability was taken into account and that detection of disturbance at a monitoring site was not impeded. A biological banding system has been developed for upland sites in the south-western Cape, together with a list of reference or expected SASS-taxa. This list includes details pertaining to seasonality and biotope preferences. The ability to define reference conditions that take intrinsic variability amongst reference sites into account is important for the accurate interpretation of bioassessment data. Handling editor: D. Dudgeon     

Techniques for the objective detection of pattern in chlorophyll samples at different temporal scales: an example using data from Lough Hyne, a seasonally stratified inlet

Chlorophyll concentrations in coastal systems are frequently variable to the extent that identifying the scales where pattern occurs is very difficult. Judgements on the temporal structure of data sets are frequently rather subjective. By examining the temporal structure of chlorophyll variation in Lough Hyne with hierarchical techniques, it was possible to identify the important temporal scales objectively. Suggestions can also be made about appropriate sampling programmes for similar coastal systems. There was significant variation in measured chlorophyll concentrations between seasons and between months within seasons. High chlorophyll concentrations were more likely during spring and autumn, as would be predicted from the seasonal cycle of stratification in the lough. Seasonality could also be detected in the tidal inflow to the lough from adjacent coastal waters. More intensive sampling during the summer did not reveal any temporal structure in surface water samples. However, there were 14 day periodicities associated with measurements of depth integrated chlorophyll, oxygen, salinity, water column stability and attenuation coefficient. It is suggested that these periodicities are consistent with spring neap tidal forcing. No interannual variation in chlorophyll concentrations was detected. Examination of the power in the analysis of variance suggested that the monthly sampling frequency was unlikely to have detected differences of less than 145% between the means of pairs of years. A sampling interval of less than a week would be needed to have confidence in detecting differences of 50% between annual means.