Joachim Maes

The impact of water hyacinth ( Eichhornia crassipes) in a eutrophic subtropical impoundment (Lake Chivero, Zimbabwe). II. Species diversity

We compared abundance and diversity of phytoplankton, zooplankton, and fishes among limnetic (P: always without macrophytes) and littoral habitats with (L +) and without (L- ) hyacinths in Lake Chivero, a man-made hypertrophic reservoir near Harare (Zimbabwe). In addition, the littoral macrophyte community, and macro-inver- tebrates associated with hyacinth mats were inventoried. The phytoplankton commu- nity was dominated by blue-green algae (mainly Microcystis aeruginosa ), typical for a hyper-eutrophic lake. Total absolute densities were about 10 to 30 times higher at the L+ sites than at the unvegetated L- and P sites. On the basis of relative species abun- dances the L- zones were more similar to the P than to the L + zones. There was an in- creasing importance of chlorophytes ( Staurastrum sp. and Pandorina morum ) and di- atoms (Cyclotella meneghiniana and pennales) and a decreasing dominance of Mycro- cystis along the discriminant axis from L +, L- to P. The zooplankton community was most dense in the unvegetated zones. Daphnids and bosminids were more abundant in the pelagic than in both littoral zones. Calanoids and Diaphanosoma were dominantly represented in the unvegetated zones. The two littoral zones were characterised by higher densities of chydorids, while they could be discriminated by the dominance of cyclopoids in the vegetated site. Seventeen different fish species were captured by at least one of the different fishing methods. Apparent habitat preferences differed ac-

Organometallic complexes for nonlinear optics. 14. Syntheses and second-order nonlinear optical properties of ruthenium, nickel and gold σ-acetylides of 1,3,5-triethynylbenzene: X-ray crystal structures of 1-(HC≡C)-3,5-C6H3(trans-C≡CRuCl(dppm)2)2 and 1,3,5-C6H3(C≡CAu(PPh3))3

The complexes 1-(HC≡C)-3,5-C 6 H 3 [M] 2 ([M] = trans -C≡CRuCl(dppm) 2 ( 1 ), C≡CNi(PPh 3 )( η 5 -C 5 H 5 ) ( 2 )) and 1,3,5-C 6 H 3 (C≡CAu(PPh 3 )) 3 ( 3 ) have been prepared by extensions of well-established σ -acetylide synthetic methodologies, and 1 and 3 have been structurally characterized. Steric restrictions preclude coordination of three trans -RuCl(dppm) 2 or Ni(PPh 3 )( η 5 -C 5 H 5 ) moieties about the central 1,3,5-triethynylbenzene unit, a problem which the smaller Au(PPh 3 ) group does not experience. The quadratic optical nonlinearities at 1.06 μ m have been investigated by hyper-Rayleigh scattering with the C 2v symmetry complexes 1 and 2 having significantly larger β HRS than their monometallic [M]C≡CPh ([M] = trans -RuCl(dppm) 2 ( 4 ) and Ni(PPh 3 )( η 5 -C 5 H 5 ) ( 5 )) counterparts. The D 3h symmetry octopolar complex 3 has a similar nonlinearity to its dipolar monometallic analogue Au(C≡CPh)(PPh 3 ) ( 6 ). Complexes 1 , 2 and 3 have λ max similar to 4 , 5 and 6 (i.e., no loss of optical transparency is observed), but have significantly larger oscillator strengths for the important optical transitions.

Organometallic complexes for nonlinear optics. 15. Molecular quadratic hyperpolarizabilities of trans-bis{bis(diphenylphosphino)methane}ruthenium σ-aryl- and σ-pyridyl-acetylides: X-ray crystal structure of trans-[Ru(2-CCC5H3N-5-NO2)Cl(dppm)2]

The complexes trans -[Ru(2-CCC 5 H 3 N-5-R)Cl(dppm) 2 ] [R=H ( 6 ), NO 2 ( 7 )] have been prepared and 7 has been structurally characterized; comparison with the structural study of trans -[Ru(4-CCC 6 H 4 NO 2 )Cl(dppm) 2 ] ( 3 ) reveals a decrease in Ru–C(1) distance and increase in Ru–Cl(1) distance, consistent with stronger σ -bonding by the nitropyridylalkynyl ligand in 7 compared to the nitrophenylalkynyl ligand in 3 . Electrochemical data for 3 , 6 , 7 , precursor dichloro complexes [RuCl 2 (dppm) 2 ] [ cis ( 1a ), trans ( 1b )], and related alkynyl complexes trans -[Ru(4-CCC 6 H 4 R)Cl(dppm) 2 ] [R=H ( 2 ), 4-C 6 H 4 NO 2 ( 4 ), ( E )-4-CHCHC 6 H 4 NO 2 ( 5 )] have been determined by cyclic voltammetry. Introduction of nitro substituent in progressing from 2 to 3 or 6 to 7 leads to a substantial increase in E o Ru II/III , most of which is lost on chain-lengthening the alkynyl ligand in progressing from 3 to 4 or 5 . Replacement of phenyl by pyridyl in progressing from 2 to 6 or 3 to 7 results in a 0.1 V increase in E o Ru II/III . The optical spectra of representative complexes have been examined. Introduction of a nitro substituent onto the phenylalkynyl ligand, in progressing from 2 to 3 , results in a substantial (ca. 11 000 cm −1 ) red-shift in the intense MLCT band of the Ru II –CC–Ar–NO 2 moiety. Complexes 3 , 4 and 5 exhibit sizable solvatochromic shifts, suggestive of significant optical nonlinearities. Quadratic hyperpolarizabilities for 2 – 7 were determined by hyper-Rayleigh scattering (HRS) at 1064 nm; measurements are consistent with an increase in β HRS upon incorporation of nitro substituent (progressing from 2 to 3 or 6 to 7 ), chain-lengthening the alkynyl ligand (progressing from 3 to 4 and then to 5 ) and replacing phenyl by pyridyl in progressing from 2 to 6 , general trends that are maintained with the two-level-corrected data, and which parallel shifts in λ max to low energy. The observed and two-level-corrected β HRS values for 7 are lower than expected; it is perhaps significant that λ max for 7 is close to the second-harmonic.

Organometallic complexes for nonlinear optics: Part 21. Syntheses and quadratic hyperpolarizabilities of alkynyl complexes containing optically active 1,2-bis(methylphenylphosphino)benzene ligands

Abstract The preparation of the chloro complex trans- [FeCl 2 {( R , R )-diph} 2 ] ( 1 ) and the alkynyl complexes trans- [M(4-CCC 6 H 4 R)Cl{( R , R )-diph} 2 ] [M=Fe, R=NO 2 ( 2 ); M=Ru, R=H ( 4 ), NO 2 ( 5 ), ( E )-CHCH-4-C 6 H 4 NO 2 ( 6 ); M=Os, R=NO 2 ( 7 )], incorporating the optically active diphosphine 1,2-bis(methylphenylphosphino)benzene (diph), are described. Oxidation potentials, as determined by cyclic voltammetry, increase as 2 7 5 . Molecular quadratic nonlinearities by hyper-Rayleigh scattering at 1064 nm increase upon introduction of an acceptor group ( 4 5 ), chain-lengthening of bridging group ( 5 6 ), and proceeding from 3d to 4d and 5d metal ( 2 ≤ 5 ≤ 7 ). Two-level-corrected nonlinearities reproduce the first two trends, but metal variation follows the sequence 2 7 5 . The experimental and two-level-corrected nonlinearities for 6 (2795×10 −30 and 406×10 −30 esu, respectively), are amongst the largest observed thus far for organometallic complexes. Crystals of complexes 2 and 7 exhibit second-harmonic generation (assessed using the Kurtz powder technique), with an efficiency for the former of twice that of urea.

The predation impact of juvenile herring Clupea harengus and sprat Sprattus sprattus on estuarine zooplankton

The consumption of estuarine copepods by juvenile herring and sprat during estuarine residency was estimated using fish biomass data and daily rations calculated from two models of feeding in fish: a bioenergetic model and a gastric evacuation model. The bioenergetic model predicted daily rations that were, on average, three times higher than those estimated by a model based on field records of stomach contents. The biomass of herring and sprat in the estuary was negatively correlated with the daily ration suggesting that the clupeid fish populations were resource-limited. Copepod production decreased towards the winter and peaked in spring and summer. The relative importance of predation changed seasonally in function of the migration pattern of herring and sprat. In the spring and the summer, in situ production of␣copepod biomass was higher than the in situ consumption by fish. During the fall and the winter, consumption exceeded production. This suggests that top–down control exerted by marine pelagic fish may be an important force structuring estuarine copepod populations.

A zone-specific fish-based biotic index as a management tool for the Zeeschelde estuary (Belgium)

Fish-based indices monitor changes in surface waters and are a valuable aid in communication by summarising complex information about the environment (Harrison and Whitfield, 2004). A zone-specific fish-based multimetric estuarine index of biotic integrity (Z-EBI) was developed based on a 13 year time series of fish surveys from the Zeeschelde estuary (Belgium). Sites were pre-classified using indicators of anthropogenic impact. Metrics showing a monotone response with pressure classes were selected for further analysis. Thresholds for the good ecological potential (GEP) were defined from references. A modified trisection was applied for the other thresholds. The Z-EBI is defined by the average of the metric scores calculated over a one year period and translated into an ecological quality ratio (EQR). The indices integrate structural and functional qualities of the estuarine fish communities. The Z-EBI performances were successfully validated for habitat degradation in the various habitat zones.

Poor water quality constrains the distribution and movements of twaite shad Alosa fallax fallax (Lacépède, 1803) in the watershed of river Scheldt

Worldwide, river fragmentation is primarily responsible for the decline of populations of migrating fish. In particular, anadromous fish species, which necessarily migrate to fresh water to reproduce, are endangered since many are no longer able to reach their natural spawning sites. In addition, pollution of rivers effectively prevents upstream or downstream movements and blocks access to spawning grounds. This article investigates how poor water quality interferes with the life history cycle of twaite shad Alosa fallax fallax (Lacépède, 1803), an anadromous clupeid fish, in the watershed of River Scheldt, a heavily impacted environment in West Europe. We used two models based on known ecological and environmental information to explain past and present twaite shad distribution within the watershed and to make inferences about a future population recovery and juvenile habitat value. We demonstrated that historical spawning areas satisfy water quality conditions necessary to support spawning and successful development of early life history stages of the twaite shad. However, poor water quality conditions just upstream the freshwater–saltwater boundary still act as an effective migration barrier for upstream movement. As a consequence, spawning grounds are inaccessible and the population is dominated by seasonal adults occurring in the lower estuarine part of the watershed. This article provides testable and diagnostic information to the watershed management in that it identifies habitat and water quality requirements needed to support the expected recovery of an endangered anadromous fish population.

Size structure and feeding dynamics in estuarine clupeoid fish schools: field evidence for the school trap hypothesis

Abstract An analysis of 273 samples taken at the filter screens of an estuarine power plant cooling water intake showed that herring and sprat were strongly associated with each other and formed mixed species schools. Herring-dominated schools were typically larger, more frequent and contained larger-sized individuals than schools dominated by sprat. The mean body length of both herring and sprat was biased as a function of their relative importance in the school suggesting that the fish schools were assorted by size. However, switching to a fish school dominated by the other species increased the individual body length variation relative to the group average. Diets of both species overlapped strongly but the feeding rate of sprat was significantly lower than that of herring. Our results suggest that during estuarine residency young sprat suffer the consequences of the school trap, possibly to profit from reduced predation risks in herring schools.

Potential re-establishment of diadromous fish species in the River Scheldt (Belgium)

This study (2002) documents on the inland penetration off diadromous fish species into the tidal and non-tidal part of the River Scheldt and on the impact of two lock-weir complexes. Long-term trends in oxic conditions show the river is undergoing a natural recovery process, nevertheless five of ten diadromous species recorded were still restricted to the brackish part of the estuary. Despite poor conditions, five species reached the upper freshwater part of the estuary. Erratic free-flowing conditions at the tidal weir offered limited opportunities for some diadromous species to migrate into the non-tidal part of the river. Upstream migration over the second barrier is almost completely blocked. Rehabilitation schemes to restore self-sustaining populations of diadromous fish in the cross-border River Scheldt need to include the building of fish bypasses and improved wastewater treatment and habitat restoration programmes.