The effect of habitat and environmental history on otolith chemistry of barramundi Lates calcarifer in estuarine populations of a regulated tropical river

We examine the microchemistry of otoliths of cohorts of a fished population of the large catadromous fish, barramundi Lates calcarifer from the estuary of a large tropical river. Barramundi from the estuary of the large, heavily regulated Fitzroy River, north-eastern Australia were analysed by making transects of ⁸⁷Sr/⁸⁶Sr isotope and trace metal/Ca ratios from the core to the outer edge. Firstly, we examined the Sr/Ca, Ba/Ca, Mg/Ca and Mn/Ca and ⁸⁷Sr/⁸⁶Sr isotope ratios in otoliths of barramundi tagged in either freshwater or estuarine habitats that were caught by the commercial fishery in the estuary. We used ⁸⁷Sr/⁸⁶Sr isotope ratios to identify periods of freshwater residency and assess whether trace metal/Ca ratios varied between habitats. Only Sr/Ca consistently varied between known periods of estuarine or freshwater residency. The relationships between trace metal/Ca and river flow, salinity, temperature were examined in fish tagged and recaptured in the estuary. We found weak and inconsistent patterns in relationships between these variables in the majority of fish. These results suggest that both individual movement history within the estuary and the scale of environmental monitoring were reducing our ability to detect any patterns. Finally, we examined fish in the estuary from two dominant age cohorts (4 and 7 year old) before and after a large flood in 2003 to ascertain if the flood had enabled fish from freshwater habitats to migrate to the estuary. There was no difference in the proportion of fish in the estuary that had accessed freshwater after the flood. Instead, we found that larger individuals with of each age cohort were more likely to have spent a period in freshwater. This highlights the need to maintain freshwater flows in rivers. About half the fish examined had accessed freshwater habitats before capture. Of these, all had spent at least their first two months in marine salinity waters before entering freshwater and some did not enter freshwater until four years of age. This contrasts with the results of several previous studies in other parts of the range that found that access to freshwater swamps by larval barramundi was important for enhanced population productivity and recruitment.     

Stormwater plume detection by MODIS imagery in the southern California coastal ocean

Stormwater plumes in the southern California coastal ocean were detected by MODIS-Aqua satellite imagery and compared to ship-based data on surface salinity and fecal indicator bacterial (FIB) counts collected during the Bight'03 Regional Water Quality Program surveys in February–March of 2004 and 2005. MODIS imagery was processed using a combined near-infrared/shortwave-infrared (NIR-SWIR) atmospheric correction method, which substantially improved normalized water-leaving radiation (nLw) optical spectra in coastal waters with high turbidity. Plumes were detected using a minimum-distance supervised classification method based on nLw spectra averaged within the training areas, defined as circular zones of 1.5–5.0-km radii around field stations with a surface salinity of S < 32.0 (“plume”) and S > 33.0 (“ocean”). The plume optical signatures (i.e., the nLw differences between “plume” and “ocean”) were most evident during the first 2 days after the rainstorms. To assess the accuracy of plume detection, stations were classified into “plume” and “ocean” using two criteria: (1) “plume” included the stations with salinity below a certain threshold estimated from the maximum accuracy of plume detection; and (2) FIB counts in “plume” exceeded the California State Water Board standards. The salinity threshold between “plume” and “ocean” was estimated as 32.2. The total accuracy of plume detection in terms of surface salinity was not high (68% on average), seemingly because of imperfect correlation between plume salinity and ocean color. The accuracy of plume detection in terms of FIB exceedances was even lower (64% on average), resulting from low correlation between ocean color and bacterial contamination. Nevertheless, satellite imagery was shown to be a useful tool for the estimation of the extent of potentially polluted plumes, which was hardly achievable by direct sampling methods (in particular, because the grids of ship-based stations covered only small parts of the plumes detected via synoptic MODIS imagery). In most southern California coastal areas, the zones of bacterial contamination were much smaller than the areas of turbid plumes; an exception was the plume of the Tijuana River, where the zone of bacterial contamination was comparable with the zone of plume detected by ocean color.     

An examination of the radiative and dissipative properties of deep ocean internal tides

In this study, the energy flux and energy dissipation of deep ocean internal tides are examined. Properties of the internal tide from two distinct generation regions are contrasted: the Mid-Atlantic Ridge (MAR) and the Hawaiian Ridge. Considerable differences are noted for the baroclinic energy flux, u^′p^′, radiated from each site. Radiation from the MAR is relatively rich in high modes, with an energy flux spectral peak at mode 5 and modes 10 and greater accounting for 40% of the total flux. In contrast, Hawaiian Ridge radiation is dominantly composed of modes 1 and 2, with modes 10 and greater accounting for less than 5% of the total flux. Depth integrated energy flux levels are at the MAR site, and at the Hawaiian Ridge. Despite these differences, observed turbulent dissipation rates at these sites are similar in magnitude and depth dependence. Decay scales, estimated as , range from O(100)km to . The mean decay scale based on the MAR data is 230 km, a factor of 3 smaller than at the Hawaiian Ridge site. We demonstrate that the dissipation level scales with the energy flux available in the high modes, which is comparable at both sites, rather than the total energy flux.     

Reference and current Trophic Level Index of New Zealand lakes: benchmarks to inform lake management and assessment

ABSTRACT

Knowledge of trophic status is fundamental to understanding the condition and function of lake ecosystems. We developed regression models to predict chlorophyll a concentrations (chl a) in New Zealand lakes for reference and current states, based on an existing dataset of total nitrogen (TN) and total phosphorus (TP) concentrations for 1031 lakes. Models were then developed to predict Secchi depth based on chl a and a sediment resuspension term applicable to shallow lakes. Estimates of all four Trophic Level Index (TLI) variables (chl a, TN, TP and Secchi depth) were analysed to estimate reference and current state TLI for the nationally representative sample of 1031 lakes. There was a trend of eutrophication between reference and current states, with systematic differences among lake geomorphic types. Mean chl a increased 3.5-fold (2.42?mg?m^?3 vs. 8.32?mg?m^?3) and mean Secchi depth decreased (indicating lower clarity) by approximately one-third (9.62?m vs. 6.48?m) between reference and current states. On average, TLI increased by 0.67, with the TLI increase >1 in approximately one-third (31%) of lakes. This study informs the status of lake ecosystems in NZ and provides benchmarks to guide management and assessment.     

Testing use of mitochondrial COI sequences for the identification and phylogenetic analysis of New Zealand caddisflies (Trichoptera)

We tested the hypothesis that cytochrome c oxidase subunit 1 (COI) sequences would successfully discriminate recognised species of New Zealand caddisflies. We further examined whether phylogenetic analyses, based on the COI locus, could recover currently recognised superfamilies and suborders. COI sequences were obtained from 105 individuals representing 61 species and all 16 families of Trichoptera known from New Zealand. No sequence sharing was observed between members of different species, and congeneric species showed from 2.3 to 19.5% divergence. Sequence divergence among members of a species was typically low (mean = 0.7%; range 0.0–8.5%), but two species showed intraspecific divergences in excess of 2%. Phylogenetic reconstructions based on COI were largely congruent with previous conclusions based on morphology, although the sequence data did not support placement of the purse‐cased caddisflies (Hydroptilidae) within the uncased caddisflies, and, in particular, the Rhyacophiloidea. We conclude that sequence variation in the COI gene locus is an effective tool for the identification of New Zealand caddisfly species, and can provide preliminary phylogenetic inferences. Further research is needed to ascertain the significance of the few instances of high intra‐specific divergence and to determine if any instances of sequence sharing will be detected with larger sample sizes.