Spread and status of seven submerged pest plants in New Zealand lakes

The distribution of seven submerged aquatic pest plants is reported. Lake vegetation surveys recorded pest plants in 27.9% of 344 lakes, with two species co‐occurring in 5.8%, and three species in 2.6% of lakes. Egeria densa was most frequent (15.4% of lakes), followed by Ceratophyllum demersum (9.0%), Lagarosiphon major (7.3%), and Utricularia gibba (5.5%). Spread since 2000 has continued for five pest plants, with 34 lakes invaded by U. gibba over 2004–08 alone. Early regional sites in proximity to human population centres were likely plant liberations and numerous potential founder colonies remain in garden ponds. Human activities were important for inter‐lake dispersal, with the exception of bird‐dispersed U. gibba. Significant lake associations between pest plants, and with presence of six exotic fish species, suggest common dispersal pathways and similar introduction risks. Therefore, predictions of future spread should be possible based on sources, dispersal pathways, and identifying key risk factors for lakes.     

Linear and nonlinear interactions of surface waves with bodies by a three-dimensional Rankine panel method

Linear and second-order surface wave interactions with floating and bottom-mounted bodies of realistic geometry are simulated in the time domain by a three-dimensional Rankine panel method. The fundamental stability analysis governing the propagation of transient wave disturbances on a panel mesh distributed on the free surface is carried out from first principles. The radiation condition is enforced by a dissipative beach selected to coincide with an outer annulus of panels. The fundamental physics governing the wave energy absorption is presented and the beach attributes are selected and validated for the linear and second-order problems. Computations are presented of the linear and sum-frequency second-order forces on a single and multiple truncated circular cylinders, and very good agreement is found with benchmark computations. The accuracy and efficiency of this method render it a promising candidate for the study of complex nonlinear wave induced phenomena upon offshore platforms, like springing and ringing.     

Bowers Swell: Evidence for a zone of compressive deformation concentric with Bowers Ridge, Bering Sea

Bowers Swell is a newly discovered bathymetric feature which is up to 90 m high, between 12 and 20 km wide, and which extends arcuately about 400 km along the northern and eastern sides of Bowers Ridge. The swell was first revealed on GLORIA sonographs and subsequently mapped on seismic reflection and 3.5 kHz bathymetric profiles. These geophysical data show that the swell caps an arcuate anticlinal ridge, which is composed of deformed strata in an ancient trench on the northern and eastern sides of Bowers Ridge. The trench fill beneath the swell is actively deforming, as shown by faulting of the sea floor and by thinning of the strata across the crest of the swell. Thinning and faulting of the trench strata preclude an origin for the swell by simple sediment draping over an older basement high. We considered several models for the origin of Bowers Swell, including folding and uplift of the underlying trench sediment during the interaction between the Pacific plate beneath the Aleutian Ridge and a remnant oceanic slab beneath Bowers Ridge. However, such plate motions should generate extensive seismicity beneath Bowers Ridge, which is aseismic, and refraction data do not show any remnant slab beneath Bowers Ridge. Another origin considered for Bowers Swell invokes sediment deformation resulting from differential loading and diapirism in the trench fill. However, diapirism is not evident on seismic reflection profiles across the swell. We favour a model in which sediment deformation and swell formation resulted from a few tens of kilometers of low seismicity motion by intraplate crustal blocks beneath the Aleutian Basin. This motion may result from the translation of blocks in western Alaska to the south-west, forcing the movement of the Bering Sea margin west of Alaska into the abyssal Aleutian Basin.     

Simulation of upper-ocean biogeochemistry with a flexible-composition phytoplankton model: C, N and Si cycling in the western Sargasso Sea

We report the first application of a biogeochemical model in which the major elemental composition of the phytoplankton is flexible, and responds to changing light and nutrient conditions. The model includes two phytoplankton groups: diatoms and non-siliceous picoplankton. Both fix C in accordance with photosynthesis-irradiance relationships used in other models and take up NO₃⁻ and NH₄⁺ (and Si(OH)₄ for diatoms) following Michaelis-Menten kinetics. The model allows for light dependence of photosynthesis and NO₃⁻ uptake, and for the observed near-total light independence of NH₄⁺ uptake and Si(OH)₄ uptake. It tracks the resulting C/N ratios of both phytoplankton groups and Si/N ratio of diatoms, and permits uptake of C, N and Si to proceed independently of one another when those ratios are close to those of nutrient-replete phytoplankton. When the C/N or Si/N ratio of either phytoplankton group indicates that its growth is limited by N, Si or light, uptake of non-limiting elements is controlled by the content of the limiting element in accordance with the cell-quota formulation of Droop (J. Mar. Biol. Ass. U.K 54 (1974) 825).We applied this model to the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea. The model was tuned to produce vertical profiles and time courses of [NO₃⁻], [NH₄⁺] and [Si(OH)₄] that are consistent with the data, by adjusting the kinetic parameters for N and Si uptake and the rate of nitrification. The model then reproduces the observed time courses of chlorophyll-a, particulate organic carbon and nitrogen, biogenic silica, primary productivity, biogenic silica production and POC export with no further tuning. Simulated C/N and Si/N ratios of the phytoplankton indicate that N is the main growth-limiting nutrient throughout the thermally stratified period and that [Si(OH)₄], although always limiting to the rate of Si uptake by diatoms, seldom limits their growth rate. The model requires significant nitrification in the upper 200 m to yield realistic time courses and vertical profiles of [NH₄⁺] and [NO₃⁻], suggesting that NO₃⁻ is not supplied to the upper water column entirely by physical processes. A nitrification-corrected f-ratio (f_NC), calculated for the upper 200 m as: (NO₃⁻ uptake—nitrification)/(NO₃⁻ uptake+NH₄⁺ uptake) has annual values ranging from only 0.05–0.09, implying that 90–95% of the N taken up annually by phytoplankton is supplied by biological regeneration (including nitrification) in the upper 200 m. Reported discrepancies between estimates of organic C export based on seasonal chemical changes and POC export measured at the BATS site can be almost completely resolved if there is significant regeneration of NO₃⁻ via organic-matter decomposition in the upper 200 m.     

Predicting the optical properties of the West Florida Shelf: resolving the potential impacts of a terrestrial boundary condition on the distribution of colored dissolved and particulate matter

Predicting the distribution of Inherent Optical Properties (IOPs) in the water column requires predicting the physical, chemical, biological, and optical interactions in a common framework that facilitates feedback responses. This work focuses on the development of ecological and optical interaction equations embedded in a 2D hindcast model of the shallow water optical properties on the West Florida Shelf (WFS) during late summer/fall of 1998. This 2D simulation of the WFS includes one case with a Loop Current intrusion above the 40-m isobath and one with the Loop Current intrusion in addition to a periodic terrestrial nutrient supply below the 10-m isobath. The ecological and optical interaction equations are an expansion of a previously developed model for open ocean conditions (Bissett, W.P., Carder, K.L., Walsh, J.J., Dieterle, D.A., 1999a. Carbon cycling in the upper waters of the Sargasso Sea: II. Numerical simulation of apparent and inherent optical properties. Deep-Sea Research, Part I: Oceanographic Research Papers, 46 (2), 271–317; Bissett, W.P., Walsh, J.J., Dieterle, D.A., Carder, K.L., 1999b. Carbon cycling in the upperwaters of the Sargasso Sea: I. Numerical simulation of differential carbon and nitrogen fluxes. Deep-Sea Research, Part I: Oceanographic Research Papers, 46 (2), 205–269). The expansion includes an increase in the number of elemental pools to include silica, phosphorus, and iron, an increase in the number of phytoplankton functional groups, and a redevelopment of the Dissolved Organic Matter (DOM) and Colored Dissolved Organic Matter (CDOM) interaction equations. It was determined from this simulation that while the Loop Current alone was able to predict the water column conditions present during the summer, the Loop Current alone was not enough to simulate the magnitude of optical constituents present in the fall of 1998 when compared to satellite imagery. Simulating terrestrial inorganic and organic nutrients and CDOM pulses coinciding with significant meteorological events and high freshwater pulses released from the major rivers feeding the WFS were required to accurately predict the distribution and scale of the inherent optical properties at the surface during the fall months. Modeling the in situ light field for phytoplankton growth and community competition requires addressing the CDOM optical constituent explicitly. The majority of the annually modeled CDOM on WFS was created via in situ production; however, it was also rapidly removed via advection and photochemical destruction. A pulse of terrestrial nutrient and organic color was required to simulate the dramatic changes in surface color seen in satellite imagery on the WFS. The dynamics of the biogeochemical portion of the simulation demonstrate the importance of nonstoichiometric supplies of terrestrial nutrients on the WFS to the prediction of nutrient and CDOM fluxes.     

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.     

2-D and 3-D modelling of wide-angle seismic data: an example from the Vøring volcanic passive margin

This study presents the modelling of 2-D and 3-D wide-angle seismic data acquired on the complex, volcanic passive margin of the Vøring Plateau, off Norway. Three wide-angle seismic profiles were shot and recorded simultaneously by 21 Ocean Bottom Seismometers, yielding a comprehensive 3-D data set, in addition to the three in-line profiles. Coincident multi-channel seismic profiles are used to better constrain the modelling, but the Mesozoic and deeper structures are poorly imaged due to the presence of flood basalts and sills. Velocity modelling reveals an unexpectedly large 30 km basement high hidden below the flood basalt. When interpreted as a 2-D structure, this basement high produces a modelled gravity anomaly in disagreement with the observed gravity. However, both the gravity and the seismic data suggest that the structure varies in all three directions. The modelling of the entire 3-D set of travel times leads to a coherent velocity structure that confirms the basement high; it also shows that the abrupt transition to the slower Cretaceous basin coincides in position and orientation with the fault system forming the Rån Ridge. The positive gravity anomaly over the Rån Ridge originates from the focussed and coincident elevation of the high velocity lower crust and pre-Cretaceous basement. Although the Moho is not constrained by the seismic data, the gravity modelled from the 3-D velocity model shows a better fit along the profiles. This study illustrates the interest of a 3-D acquisition of wide-angle seismic over complex structures and the benefit of the subsequent integrated interpretation of the seismic and gravity data.