Elevated biomass of mesozooplankton and enhanced fecal pellet flux in cyclonic and mode-water eddies in the Sargasso Sea

Accumulating evidence points to the importance of mesoscale eddies in supplying nutrients to surface waters in oligotrophic gyres. However, the nature of the biological response and its evolution over time has yet to be elucidated. Changes in mesozooplankton community composition due to eddy perturbation also could affect biogeochemical cycling. Over the course of two summers we sampled seven eddies in the Sargasso Sea. We focused on and followed a post-phytoplankton bloom cyclonic eddy (C1) in 2004 and a blooming mode-water anticyclonic eddy (A4) in 2005. We collected zooplankton in all eddies using a Multiple Opening and Closing Net Environmental Sampling System (MOCNESS) and quantified biomass (>0.15 mm, in five size fractions) from 0 to 700 m over nine discrete depth intervals. Zooplankton biomass (>0.5 mm) in the upper 150 m was similarly enhanced at night for the periphery of C1 and the center of A4 at 0.514 g m⁻² and 0.533 g m⁻², respectively, compared to outside (0.183 g m⁻² outside C1 and 0.197 g m⁻² outside A4). Despite minimal chlorophyll a enhancement and dominance by picoplankton in C1, zooplankton biomass increased most for the largest size class (>5 mm). Gut fluorescence for euphausiids and large copepods was also elevated on the C1 periphery. In A4, peak biomass occurred at eddy center coincident with peak primary production, but was highly variable (changing by >3-fold) over time, perhaps resulting from the dense, but patchy distribution of diatom chains in this region. Shifts in zooplankton community composition and abundance were reflected in enhancement of fecal pellet production and active transport by diel vertical migration in eddies. Inside C1 the flux of zooplankton fecal pellets at 150 m in June 2004 was 1.5-fold higher than outside the eddy, accounting for 9% of total particulate organic carbon (POC) flux. The flux of fecal pellets (mostly from copepods) increased through the summer in eddy A4, matching concurrent increases in zooplankton <2 mm in length, and accounting for up to 12% of total POC flux. Active carbon transport by vertically migrating zooplankton was 37% higher on the periphery of C1 and 74% higher at the center of A4 compared to the summer mean at the Bermuda Atlantic Time-series Study (BATS) station. Despite contrasting responses by the phytoplankton community to cyclonic and mode-water eddies, mesozooplankton biomass was similarly enhanced, possibly due to differential physical and biological aggregation mechanisms, and resulted in important zooplankton-mediated changes in mesoscale biogeochemistry.     

Water quality in a polluted lowland stream with chronically depressed dissolved oxygen: Causes and effects

Abstract

The Whangamaire Stream (North Island, New Zealand) has high concentrations of nitrate nitrogen (NO^? ₃‐N), biochemical oxygen demand (BOD₅), and Kjeldahl nitrogen (TKN) as a result of catchment land use practices. The lower reaches of the stream drain intensively farmed land and have dissolved oxygen (DO) levels of 10–50% saturation. The dominant riparian vegetation, Apium nodiflorum, provides a large organic loading by intercepting nutrients in run‐off and then decaying in the stream channel. Water quality and reaeration aspects of the stream were studied in order to explain the observed low DO levels. Measurements of the reaeration coefficient at 20°C, K₂ ²⁰, using methyl chloride (CH₃Cl) as a gas tracer, yielded values of 1.1–3.0 d^?1 for the upper part of the study reach and 15.5–16.2 d^?1 for the lower reach (overall average 12.5 ± 2.5 d^?1). These were in agreement with values inferred from single‐station diurnal curve analysis, which also showed that respiration was dominant in the lower reach where photo‐synthetic activity was inhibited by shade. The relatively large reaeration coefficients ensure that parts of the stream do not become anoxic at night time. Better riparian management and reduced nutrient inputs are likely to improve stream water quality.     

The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.     

Observations of polar patches generated by solar wind Alfvén wave coupling to the dayside magnetosphere

A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (K_p = 7- and IMF B_z <0). The ionosondes measured electron densities of up to 9 × 10¹¹ m⁻³ in the patch center, an increase above the density minimum between patches by a factor of ≈4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF B_y > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts (flow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF B_y) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfvén waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.     

Spatial variation of the intertidal sediments and macrozoo-benthic assemblages along Eighty-mile Beach, North-western Australia

The extensive intertidal flats along Eighty-mile Beach in North-western Australia appear to be monotonous and homogeneous and seem ideally suited to study tidal zonation in macrozoo-benthic communities and their possible correlates with characteristics of the sediment. In October 1999, we sampled benthic invertebrates and sediments at a total of 895 sampling stations distributed over six different locations, each location separated by 15 km of unsampled foreshore along Eighty-mile Beach. To test for the presence or absence of patterns of tidal zonation (distinct height-related zones of specific sediment grain sizes or zoobenthic taxonomic groups) or patchiness (distinct patches of specific sediment grain sizes or zoobenthic taxonomic groups not related to tidal height) each location was divided into three along-shore sections and each section (transect) was examined at two or three tidal heights. Zonation was observed for sediment grain sizes. Sediments were coarser at the highest intertidal level and finer towards the low water line. Benthic assemblages also differed among tidal heights, but in terms of species-composition the differences were not consistent among the locations. Each location supported a unique collection of benthic invertebrates. Therefore the hypothesis of the presence of distinct zones of specific species or zoobenthic taxonomic groups was rejected; the presence of benthic patches was confirmed. The distribution of sediments and the composition of benthic assemblages were surprisingly poorly correlated compared to those reported in 12 previous quantitative studies around the world. One possible explanation might be that super-cyclone Vance, which hit the study-area only six months before this study, contributed to this poor correlation. Alternatively, the poor correlation may indicate that biotic interactions are more important than the assumed abiotic structuring.     

Population structure of the sicklefin lemon shark Negaprion acutidens within the Curieuse Marine National Park,Seychelles

This study produced the first data on population parameters for sicklefin lemon shark Negaprion acutidens at the granitic Seychelles Islands, adding to a limited catalogue of species information. Juveniles at a site in the Curieuse Marine National Park were tagged over two periods, in 2014–2015 and 2015–2016, with measurements of length (precaudal, fork and total) and weight, sex, and state of the umbilical opening recorded. Recapture data were used to calculate growth rates and population size. Mean annual growth rate was 5.40 cm y^–1. A mean size at birth of 62.5 cm TL is in line with previous assessments of the species. Population-size estimates were similar for the two sampling periods (comprising 311 and 255 individuals) and higher than anticipated. Movements to outside the lagoon were recorded for early ages, and this, combined with weight loss and a reduction in condition factor as displayed in several individuals, suggests that the study area alone could not support the large number of sharks pupped yearly.     

Effects of the oophagous bivalve Acesta oophaga on the morphology and fecundity of its deep‐sea tubeworm host,Lamellibrachia luymesi

The deep‐sea bivalve Acesta oophaga lives attached to the anterior end of the vestimentiferan tubeworm, Lamellibrachia luymesi, at cold methane seeps. The bivalve is found almost exclusively on female tubeworms, where it consumes the lipid‐rich eggs of L. luymesi that are spawned year round (Biological Bulletin, 209, 2005, 87). It is apparent that A. oophaga benefits directly from this close association, but the consequences for the tubeworm host may be more complicated than just a simple predator–prey interaction. Since A. oophaga completely surrounds the tube opening and plume of the worm, it is likely that its presence would limit oxygen uptake by L. luymesi, thereby inhibiting worm growth and reproduction. We hypothesized that occupied tubeworms would compensate for this by growing larger plumes for oxygen uptake. To explore the effects of bivalve presence/absence on female tubeworms, several morphological features, including body size, plume length, tube diameter, and tube segment length, as well as instantaneous fecundity, were compared. Results suggest that the mere presence of A. oophaga has a significant impact on the morphology of its host worm, as all measures of worm size, except for tube segment length, were significantly greater with clams present. Additionally, instantaneous fecundity was 3.5 times higher in occupied worms, implying that tubeworms are not oxygen‐deprived or energy limited as a result of bivalve presence. Our findings suggest that the association between these two deep‐sea organisms may be a more complex form of symbiosis than the simple predator–prey relationship, as previously thought.     

Can turbidity caused by Corophium volutator (Pallas) activity be used to assess sediment toxicity rapidly?

The standard toxicity test organism, Corophium volutator, exhibits a behavioural response to contaminated sediments that causes increased turbidity of overlying water. We quantify the effects of this response to an estuarine sediment spiked with copper and hydrocarbon contaminated sediments from an oil installation in the North Sea. Turbidity measured 24 h after the start of a toxicity test shows a strong relationship with contaminant concentrations and with mortality after 10 days. Turbidity measurements can therefore give a rapid indication of sediment toxicity, permitting a reduction in storage time of sediments to be used in dilution series and toxicity identification evaluation (TIE) tests, reducing the likelihood of contaminants degrading prior to testing.     

Modern swathe sounding and sub-bottom profiling technology for research applications: The Atlas Hydrosweep and Parasound Systems

This chapter describes two separate but complementary research echosounder systems originally developed by Krupp Atlas Elektronik GmbH for the new German oceanographic research vessel Meteor.The Hydrosweep is a Hydrographic wide-swathe sweep survey echosounder for both shallow and deep water applications providing accurate bathymetric surveys and terrain-following navigation capabilities.The Parasound system is a hull-mounted dual channel parametric narrow-beam deep sea survey and sub-bottom profiling echosounder enabling particularly high vertical and horizontal resolution of seabed features.     

Modification of an acoustic pinger to transmit temperature data

An acoustic pinger can be modified to transmit two pings per second with the delay time between pings controlled by the temperature of an external thermistor. The precision of the temperature measurements is on the order of 0.1°C. This modification does not interfere with the normal depth indicating function of the pinger and is an inexpensive means of obtaining water temperature data.