Ecological investigations on the zooplankton community in balsfjorden,northern Norway: Lipids and fatty acids in Meganyctiphanes norvegica,Thysanoessa raschi and T. inermis during mid-winter

Total lipid of Meganyctiphanes norvegica (M. Sars) contained 53% triacylglycerols and traces of wax esters, that of Thysanoessa raschi (M. Sars) contained 44% triacylglycerols and 10% wax esters and that of T. inermis (Krøyer) contained 28% triacylglycerols and 40% wax esters. The triacylglycerols of M. norvegica were relatively rich in 20:1 and 22:1 fatty acids and its traces of wax esters resembled those of calanoid copepods. The triacylglycerols of both Thysanoessa species were deficient in 20:1 and 22:1 fatty acids but were richer in 16:1(n-7) and 18:1 (n-7) acids than those of M. norvegica. The wax esters of T. raschi contained phytol as almost the only fatty alcohol and were rich in 16:0 and 18:1 (n-9) fatty acids. The wax esters of T. inermis contained mainly 16:0 and 14:0 fatty alcohols with lesser amounts of phytol and their dominant fatty acid was 18:1, especially the (n-9) isomer. The triacylglycerols of T. inermis had 18:4 (n-3) as the major polyunsaturated fatty acid. From these and other aspects of fatty acid and fatty alcohol analyses it is concluded that a major foodstuff of M. norvegica in Balsfjorden is wax ester-rich calanoid copepods. T. raschi and especially T. inermis are concluded to have much more preference for phytoplanktonic food. Results are discussed in terms of current knowledge of the lipid chemistry of krill in the northern and southern hemispheres.     

Turnover and vertical transport of zinc by the euphausiid Meganyctiphanes norvegica in the Ligurian Sea

Participatory turnover time is defined as the time required to cycle an element in a system through a given material in that system. The participatory turnover time of ionic zinc by the adult Meganyctiphanes norvegica population in the Ligurian Sea ranged between 498 and 1243 years, depending upon the available food supply, and considering the food chain as the only route for zinc accumulation by the population. A total-impact turnover time was calculated as the sum of the participatory turnover time for live individuals plus the time required for dead euphausiids to lose 90% of their zinc to the water. Carcasses lost zinc to the water slower than either feces or molts, and so established the maximum loss time for all particulate excretion products; nevertheless, total-impact turnover time for zinc did not differ significantly from the participatory turnover time. The net vertical transport of zinc by M. norvegica from the sea surface to any specified depth can be calculated as the sum of the dissolved zinc excreted below the depth plus the concentrations of zinc left in feces, molts, and carcasses after they have sunk to the specified depth. Carcasses sink the fastest and lose the smallest fraction of their zinc concentration during descent; fecal pellets sink the slowest and lose the greatest fraction of their zinc concentration, and molts are intermediate. Nevertheless, feces represents the major route for delivering zinc to the bottom of the Ligurian Sea (2500 m), because concentration of the element in the pellets is so much higher than in carcasses or molts. Excretion of dissolved zinc into the water at the vertical migration depth of the living population during daylight hours was also inconsequential. Feces zinc represented over 80% of the total zinc transported to the sea floor if only marginal food supplies were available to the euphausiids, and over 90% if food was in sufficient supply. M. norvegica can effect a net transport of about 98% of its body zinc concentration below 500 m daily, in conditions of sufficient food supply and assuming that no released products are eaten during descent. If the food supply in the Ligurian Sea is considered only marginal throughout the year, M. norvegica can still effect a daily net transport below 500 m of about 36% of its body concentration, and about 6% of its body concentration will reach 2500 m daily.     

The functional biology and trophic role of krill (Thysanoessa raschii) in a Greenlandic fjord

Despite being a key zooplankton group, knowledge on krill biology from the Arctic is inadequate. The present study examine the functional biology and evaluate the trophic role of krill in the Godth?bsfjord (64°N, 51°W) SW Greenland, through a combination of fieldwork and laboratory experiments. Krill biomass was highest in the middle fjord and inner fjord, whereas no krill was found offshore. The dominating species Thysanoessa raschii revealed a type III functional response when fed with the diatom Thalassiosira weissflogii. At food saturation, T. raschii exhibited a daily ration of 1% body C d^?1. Furthermore, T. raschii was capable of exploiting plankton cells from 5 to 400???m, covering several trophic levels of the pelagic food web. The calculated grazing impact by T. raschii on the fjord plankton community was negligible. However, the schooling and migratory behaviour of krill will concentrate and elevate the grazing in specific areas of the euphotic zone.     

Ovarian development and spawning in relation to the moult cycle in Northern krill, Meganyctiphanes norvegica (Crustacea: Euphausiacea), along a climatic gradient

Adaptive processes linked to reproduction were studied comparatively for three populations of Northern krill, Meganyctiphanes norvegica (M. Sars, 1857), sampled during winter and summer cruises in the Clyde Sea (W Scotland), the Kattegat (E Denmark), and the Ligurian Sea (NW Mediterranean). The aim was to investigate the functional relationship between egg production and moulting under contrasted climatic and environmental conditions. A staging system for female sexual development established for live krill was complemented by a histological study of the ovary at various developmental steps. During the reproductive season, all adult female krill were engaged in cyclical egg production. During experiments, female krill released one batch of mature oocytes in one or two spawning events. The ovary of postspawn female krill still contained developing oocytes for another egg batch. In the non-reproductive period, all female krill had a resting ovary. Ovarian structure and pattern of egg production were identical in the three populations, but seasonal timing of egg production was different. The model proposed for the Ligurian population of the annual cycle of ovarian development can be extended to the other two populations, taking into account the seasonal characteristics of each site. Random field samples were staged simultaneously for moult cycle and for sexual development. Moult stages and the seasonal variation of the intermoult period were studied for the Kattegat population using multi-year data and compared to data obtained during summer/winter cruises in the Clyde and the Ligurian Sea. At the three sites, intermoult period was shorter and temperature-dependent during the reproductive period, concurrent with the season of greatest food availability. During most of the year and the period of sexual rest, moulting activity was reduced. The relationship between spawning and the moult cycle was studied comparatively for the three populations. Eggs were released during the premoult phase of a “spawning moult cycle”, in one or two spawnings associated with apolysis and Moult Stage D1, respectively. Yolk accumulation for the next egg batch was completed during an alternating “vitellogenic moult cycle”. A model for the timing of cyclical egg production in relation to moulting, as proposed for the Kattegat, can be extended to the other populations, taking into account intermoult period variation with temperature. Temperature appeared to be the principal environmental factor controlling growth (through moulting) and egg production during the reproductive season, in connection with favourable trophic conditions. Received: 22 December 1997 / Accepted: 29 August 1998     

Diel vertical migration and feeding patterns of Mysis mixta (Crustacea,Mysidacea) in the Baltic Sea

The extent of the nocturnal vertical migration of Mysis mixta Lilljeborg varied between early July and late October (of 1985 and 1986) in a coastal area of the Baltic Sea. Migration was more restricted in early July and late October. Seasonal changes in surface light levels and transparency were sufficient to explain the observed differences. Mysids avoided light levels above 10^-4 lux throughout the study period. Smaller juveniles migrated higher up than larger juveniles and adults. A two-layered distribution with part of the population close to the bottom was observed at night. Zooplankton were more abundant in water layers above the main concentration of mysids. M. mixta fed on phytoplankton, detritus, copepods, cladocerans, rotifers and tintinnids. Diel changes in gut fluorescence indicated a higher intake of phytoplankton at night, but levels were low compared to primarily herbivorous zooplankton. Comparisons of stomach contents of mysids caught at the bottom in the evening and in the water column at night showed a higher ingestion of zooplankton at night and of detritus during the day. Mysids caught at the bottom at night had an intermediate diet. Copepods and cladocerans constituted between 90 and 100% of ingested material by weight in all mysid groups.     

Genetic differentiation in a pelagic crustacean (Meganyctiphanes norvegica: Euphausiacea) from the North East Atlantic and the Mediterranean Sea

Meganyctiphanes norvegica (M. Sars)is a pelagic crustacean that plays a key role in marine food webs of North Atlantic Ocean and marginal seas. We studied eight population samples collected in the European Atlantic and Mediterranean Sea. By means of single strand conformation polymorphism analysis (SSCP) and direct sequencing, we investigated a segment of 158 base pairs of the mitochondrial gene coding for the subunit 1 of NADH dehydrogenase. We found 12 sequence variants among the 385 individuals studied. Analysis of molecular variance (AMOVA) showed that 14.75% of the total genetic variability was explained by differences between populations, thus indicating absence of panmixia for these populations. Pairwise comparisons revealed three distinct genetic pools: the first one represented by Cadiz Bay, the second one by the Ligurian Sea, and the third one included all the NE Atlantic samples. We also investigated one population from the Alboran Sea (within the Mediterranean basin, east of the Strait of Gibraltar). This population was found to be genetically intermediate between the NE Atlantic samples and the Ligurian sample, suggesting that the restriction to the gene flow is not associated with the Strait of Gibraltar, but possibly with the Oran–Almeria oceanographic front. The present work indicates that M. norvegica, although endowed with a high dispersal capacity because of its pelagic habit, can develop separate breeding units inside the same oceanic basin (the Atlantic). Furthermore, the Ligurian sample should be considered as a distinct evolutionary entity, separated from the Atlantic population. Received: 2 May 1999 / Accepted: 26 November 1999     

Mesozooplankton community structure in offshore and coastal waters of the Ionian Sea (eastern Mediterranean) during mixed and stratified conditions

Depth-stratified samples, collected during a period where the water column was vertically mixed (March 2000) and a period of thermal stratification (September 2000), were analyzed in order to investigate the horizontal and vertical distribution patterns and composition of mesozooplankton, especially copepod species assemblages, in a pelagic (Ionian Sea) and a coastal area (Patraikos Gulf) of the eastern Mediterranean. Total mesozooplankton abundance and biomass were significantly lower in the highly oligotrophic offshore waters of the Ionian Sea when compared to the semi-enclosed Patraikos Gulf during both seasons. Small-sized copepods dominated the mesozooplankton community. An ‘offshore’ and a ‘coastal’ copepod assemblage were defined in the surface layer (0–50 m) only during March when differences in environmental conditions (i.e., temperature, salinity and fluorescence) were strong between the two areas. Copepod vertical community structure in offshore waters differed between sampling months. In March one assemblage (0–200 m) was mainly identified, while in September three distinct assemblages (0–50, 50–100 and 100–200 m) were observed, related to different vertical distribution patterns of the various copepod species. A pronounced seasonal change of the dominant copepods was evident in the surface layer, where strong differences in hydrological properties were observed from March to September. Below this layer, the copepod community was relatively stable showing decreasing seasonal differences with increasing depth.     

Daytime surface swarming of Euphausia pacifica (Crustacea: Euphausiacea) in the Sanriku coastal waters of northeastern Japan

Biological characteristics of daytime surface swarming of Euphausia pacifica were investigated. Swarms in the Sanriku coastal waters consisted almost exclusively of mature individuals and seemed to be related to breeding behavior. Although maturation of ovarian eggs occurred both at the beginning and the end of the swarming period, females with attached spermatophore were most frequent at the end of the period. Since stomachs of swarming individuals were nearly empty throughout this period, swarming behavior was probably not related to feeding. Swarming individuals did not seem to be transported to the surface by vertical water movement caused by tidal streams, because the daily catch per boat did not differ between spring and neap tides.     

Movements of tiger sharks (Galeocerdo cuvier) in coastal Hawaiian waters

The giant clam Tridacna crocea harbors in the mantle tissue symbiotic microalgae commonly called zooxanthellae. Isolated zooxanthellae release glycerol into the medium in the presence of mantle tissue homogenate (MH), but it is not clear whether the cells do so in situ. In order to determine the photosynthetic products released by zooxanthellae in the mantle of the giant clam we traced photosynthetic fixation products from ¹³C- and ¹⁴C-bicarbonate both in the clam and in isolated zooxanthellae (IZ) in the presence or absence of MH. After 15 min incubation in the absence of MH the IZ released less than 0.6% of the fixed labeled carbon, mainly as glucose. The major intracellular photosynthates were neutral lipids, which constituted 20 to 40% of the total extractable ¹⁴C. In the presence of MH, the IZ released up to 5.6% of the total fixed ¹⁴C, mostly as glycerol, and the major intracellular photosynthate was glucose. In an intact clam incubated in sea water containing ¹⁴C-bicarbonate, 46 to 80% of the fixed ¹⁴C was translocated from the zooxanthellae to the host tissues. Most of the ¹⁴C in the hemolymph, in the isolated zooxanthellae and in intact mantle tissue (containing zooxanthellae) was recovered as glucose. No ¹⁴C-glycerol was detected in the mantle after 1 to 30 min incubation, and, even after 60 min, far less ¹⁴C-glycerol was synthesized than by IZ in the presence of MH. The possibility that in clam tissue glycerol is converted to glucose was examined by tracing the labeled carbon from ¹⁴C-glycerol injected into the adductor muscle. After 5 min incubation, no labeled glucose was found in the hemolymph, but after 60 min, some 20% was found as glucose. Thin slices containing zooxanthellae, cut from the surface of the mantle, fixed inorganic carbon supplied as NaH¹⁴CO₃ in the medium and mainly released ¹⁴C-glucose. The addition of MH to the surrounding medium did not affect the release rate or form of release product. When the slices were cut into smaller pieces, however, the ratio of glycerol to glucose in the release product increased. These results indicate that in the presence of MH the metabolism of isolated zooxan- thellae was different from that of zooxanthellae in the mantle. In the presence of MH, isolated zooxanthellae release mostly glycerol, whereas in the mantle they release glucose. Received: 18 February 1998 / Accepted: 4 December 1998     

Life cycle, seasonal vertical distribution and feeding of Calanus finmarchicus in Skagerrak coastal water

 Day/night zooplankton sampling in Kosterfjorden, Sweden, gave information on population succession, vertical distribution and feeding of Calanus finmarchicus over 17 months. Copepodid Stage 1 and 2 (C-I, C-II) were present from December to August, indicating reproduction during most of the year. Mating and breeding for the overwintering generation mainly took place in February/March, resulting in peak abundance of C-I in March/April. Secondary breeding periods were in April and July/August, but the resulting recruitment from these were low. The relative recruitment success in the first spring was around 13 times higher than during the second spring. Low temperature in the surface water early in the year and depression of the spring phytoplankton bloom may have caused the failure in the second year. Population biomass peaked at >15 g dry weight m⁻² during the numeric peak of the youngest stages in March/April. A more sustainable level of high biomass of 8 to 10 g dw m⁻² was gradually built up during summer, mainly due to a continuous accumulation of C-Vs. Adults and C-Vs comprised the overwintering population, with 7 to 14% and 85 to 93%, respectively, for the 2 years, but only C-Vs staying in the deep water were in a resting state. Adult males showed a strong diurnal vertical migration (DVM) of the usual type from spring to early autumn and a reversed DVM during the cold season. They were in a feeding state throughout the year. Adult females showed the usual type of DVM during summer to autumn, but commonly a reversed DVM during winter to spring. They were usually in a feeding state, with no pronounced differences between surface and deep water or between day and night. C-Vs aggregated in the deep water from October to March and performed DVM in April to June. They were commonly feeding in the deep water between March and June but showed no or very low feeding activity there from July to February. C-Vs in the surface water were commonly feeding and showed the highest proportion of feeding in autumn, when the population in the deep water was inactive. C-V constituted up to nearly 100% of the population biomass, and therefore must be of profound ecological importance. Defined by this dominant role, the population of C. finmarchicus can be characterised as having an active period of feeding, reproduction and development from February to July with a following 6 to 7 months of resting in the deep water, when development is arrested and no feeding occurs. Received: 1 October 1999 / Accepted: 27 April 2000     

Brevetoxins in sharks and rays (Chondrichthyes, Elasmobranchii) from Florida coastal waters

In October 2000, a mass mortality of blacktip sharks (Carcharhinus limbatus) and Atlantic sharpnose sharks (Rhizoprionodon terraenovae) in northwest Florida occurred in conjunction with a Karenia brevis red tide bloom. Before this incident, no information existed on red tide-induced shark mortalities or baseline brevetoxin levels in sharks and rays from red tide-endemic areas. We report here that brevetoxin accumulation in live and red tide-killed elasmobranchs is common during K. brevis blooms and non-bloom periods. Strong relationships were found between the frequency of red tide blooms and the average brevetoxin concentrations in elasmobranch tissues. The presence of brevetoxins in Atlantic coast sharks in the absence of documented K. brevis blooms may suggest that blooms are occurring in areas that are not well monitored. Although red tide-related shark mortalities are rarely observed, the presence of brevetoxins in shark embryos raises questions about the effects these toxins may have on the reproductive success of sharks.     

Diel variation of TCO2 in the upper layer of oceanic waters reflects microbial composition,variation and possibly methane cycling

Six diel TCO₂ cycles determined by infrared (IR) photometry from five drift stations occupied between 24 February and 16 March 1979 in the mixed layer of the northwestern Caribbean Sea are examined. Comparison of TCO₂ variation with coincident salinity and O₂ variation demonstrated that TCO₂ often co-varied with these independently measured variables. During five diel cycles TCO₂ variation was characterized by nocturnal production and diurnal consumption. The inverse, diurnal production of CO₂, occurred downstream from Misteriosa Bank, whose corals apparently contributed to a water mass having a twofold increase of POC and a sixfold larger population of heterotrophic nanoplankters. For the five diel studies carried out in waters with balanced or nearly blanced heterotrophic and phototrophic components of the nanoplankton, CO₂ consumption at constant salinity always occurred between 06.00 and 09.00 hrs. Net uptake often continued through 15.00 hrs, but not always in the absence of significant salinity changes. At constant salinity net O₂ evolution never exceeded 0.5 mol l^-1 h^-1 while net CO₂ uptake consistently averaged 3 mol l^-1 h^-1 for an apparent net production of 36 mg C m^-3 h^-1, which greatly exceeds the O₂ changes and open ocean ¹⁴C estimates from the literature. Diurnal consumption was apparently balanced by nocturnal production of CO₂ so that no significant net daily change in TCO₂ was observed. Departures from theoretical PQ and RQ and the possibility of nocturnal variations in formaldehyde and carbonate alkalinity imply that chemotrophs, both methane producers and methane oxidizers, play a significant role in CO₂ cycling. This could be through the metabolism of the nonconservative gases CH₄, CO, and H₂, and a link between chemotrophy and phototrophy through these gases is hypothesized. These open system measurements were subject to diffusion and documentable patchiness, but temporal TCO₂ changes appear to indicate the net direction of microbiological activity and join a growing body of literature showing dynamic variation in CO₂ and O₂ that exceeds estimates by ¹⁴C bottle assays of carbon fixation.     

Lunar rhythms of vertical migrations coded in otolith microstructure of North Atlantic lanternfishes,genus Hygophum (Myctophidae)

Otoliths of five Hygophum species were examined by means of light and scanning electron microscopy. In otoliths of four species (H. benoiti, H. macrochir, H. reinhardtii and H. taaningi) a strong cyclic pattern of the incremental structure was observed. In the fifth species (H. hygomii) such a pattern did not exist. An analysis of archival data on mesopelagic collections suggested three types of Hygophum spp. migratory behavior in relation to the lunar cycle which corresponded with the otolith microstructure. In H. hygomii only limited influence of moon phase on the uppermost range of night-vertical migration toward the surface was observed. The abundant nighttime occurrence of this species moved from the 0 to 50 m into the 50 to 100 m depth strata at full moon. In H. benoiti, a great part of the population, mainly juveniles, showed a tendency toward cessation of the vertical migrations during the first and fourth quarters of the lunar cycle. H. macrochir and H. taaningi had the strongest correlation of behavior with the lunar cycle. Both species showed arrested vertical migrations at the new moon phase, staying at day depths during the night, i. e., below 400 m. Thus, sequences of clear growth increments in otoliths represented a fast-growth period associated with the night migration to the warm surface layers, while bands without easily distinguishable incremental structure were interpreted as a period of slow growth in deep, cold waters due to limitation of the upward migration range occurring approximately at new moon.     

Horizontal and vertical trends in the distributions of larval fishes in coastal waters off central New South Wales,Australia

Larval fishes were sampled across six transects perpendicular to a 50 km section of the coast off Sydney, Australia, in April/May and August/September 1990. Samples were collected at the surface and at depth (20 to 30 m) at three locations across each transect; over the 30, 70 and 100 m depth contours. There was a large level of heterogeneity in the horizontal and vertical distributions of most taxa examined, and no general pattern of distribution was evident for the whole assemblage. Classification analyses revealed that the major differences between assemblages were related to depth. Horizontal trends in the distributions of the abundant taxa were evident in the inshore-offshore direction, but not longshore. Seven taxa belonging to the families Gobiidae, Labridae, Sillaginidae, Sparidae, Ambassidae, Clupeidae and Clinidae/Tripterygiidae were most abundant inshore, whereas 4 taxa of the families Cepolidae, Percichthyidae, Cheilodactylidae and Gonorynchidae were generally more abundant offshore and 24 taxa showed no discrete horizontal trends across transects. More taxa and individuals were generally caught at depth than at the surface and this was evident across all transects. Twenty taxa were more numerous at depth, whereas 4 taxa, the Cheilodactylidae, Gonorynchidae, Mullidae and Scorpididae, were most abundant at the surface and 11 taxa showed no difference in densities between depths. Ontogenetic differences in the distributions of some larvae were evident. The mean size of larval Liza argentea (Mulgilidae) caught was greater offshore than inshore, and greater at the surface than at depth. In contrast, larger Pseudocaranx dentex (Carangidae) occurred in greater numbers at depth than at the surface. The data emphasise that the assemblages of larval fishes in coastal waters off central New South Wales cannot be modelled as a single unit, which concurs with the findings in other temperate and tropical vaters. Furthermore, the data denote the need to spatially stratify sampling in these waters in order to assess seasonal changes in these assemblages.     

Effect of microclimatic factors and tide on vertical migrations of the mangrove crab Sesarma leptosoma (Decapoda: Grapsidae)

The tree crab Sesarma leptosoma Hilgendorf migrates up the mangrove trunks twice a day, both in the morning and in the evening, leaving its refuge near the tree base to feed on fresh leaves. In spite of the 150 cm of tide excursion at the base of the trees, the timing of the migrations seems largely controlled by nontidal factors. The timing of the morning upward migration is controlled by the light level alone. The morning downward migration takes place as soon as the temperature rises and relative humidity falls, with no relationship to tidal cycle or light level. Only the evening upward flow seems primarily controlled by the tide, while the evening downward flow, which ends at twilight, seems to be related to the timing of the evening upward migration, to the tide and again to light levels. During the day, crabs abandon the canopy to avoid low relative humidity, usually coupled with the higher wind speeds, which would quickly dehydrate them. At night, crabs stop migrating, probably because their movements and orientation capability along the trunk and in the canopy are largely vision dependent. Nontidal factors have never previously been shown to influence the migratory behaviour of an intertidal crustacean species so extensively. Received: 14 June 1997 / Accepted: 17 July 1997     

Mercury in surface waters of a macrotidal urban estuary (River Adour, south-west France)

Surface waters were collected in the River Adour estuary (south-western France) during different sampling periods from 1998 to 2001 in order to investigate the phase distribution and speciation of methylmercury (MeHg⁺). Although a high variability is observed, significant higher average concentrations of total MeHg⁺ (dissolved and particulate) are measured during the warm seasons, at 4.40±8.18 pM and 3.90±1.87 pM for July 1998 and September 1999, compared with the winter seasons, with concentrations at 0.99±2.85 pM and 1.00±1.75 pM for February 1998 and February 2001, respectively (one-tailed t-test, P=0.01). The seasonal variations are explained with enhanced bacterial activity during summer and sedimentation/resuspension phenomena. Additionally, signi-ficant longitudinal variations of the MeHg⁺ concentrations are observed. The highest levels in both dissolved and particulate fractions are found within the downstream urban estuarine area. This can be explained by the high methylation potential of the sediments, but direct anthropogenic inputs of MeHg⁺ from specific discharge points cannot be neglected. Biogeochemical factors like phytoplankton biomass and salinity also show a relationship with MeHg⁺ partitioning in the surface waters of the Adour estuary.     

Diel variability of feeding activity in haddock (Melanogrammus aeglifinus) larvae in the East Shetland area, North Sea

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock (Melanogrammus aeglifinus) larvae four times per day during a 10-d cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate to high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (19:00 hrs) and peak digestive enzyme activity at 01:00  hrs. The time scale over which enzyme activities reacted to prey ingestion was ca. 6 h, and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e.g. food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons. Received: 29 October 1998 / Accepted: 18 June 1999     

Water quality in the Red Sea coastal waters (Egypt): Analysis of spatial and temporal variability

In order to provide a background picture of the water quality of the Egyptian Red Sea a number of hydrological and chemical parameters have been measured bimonthly in 2000. Few data are available on this area, which is apparently subjected to an increasing human impact due to recreational (swimming and diving), industrial (mainly phosphate shipping and industry) and fishing/harbor activities. The results of the present study indicate that changes in the salinity and pH were not significant with highly oxygenated seawaters. The levels of suspended solids (as total suspended matter, TSM) and chlorophyll-a (Chl-a) were generally low and showed an homogeneous distribution in the study region. The ratio of chlorophyll-a to total suspended matter concentrations increased between November and March and decreased from May to September. Chlorophyll-a was significantly correlated with transparency and total suspended matter concentrations in July, September and November. Nitrogen, phosphorus and reactive silicate concentrations were generally low, and allowed classifying the Egyptian Red Sea coastal water as oligotrophic to mesotrophic. The middle region of the study area, which was located between Safaga and Qusair displayed relatively high phosphate contents when compared with other coastal areas. The high values of N:P ratios indicate that PO 4 -P is the limiting factor for phytoplankton growth in the Red Sea coastal waters, with the possible exception of the middle region. Significant relationships were found between chlorophyll-a concentrations and nutrient levels in different sampling periods. Spatial distribution patterns of the studied variables revealed that productivity of the Red Sea coastal waters is mostly controlled by phosphate concentrations, salinity, temperature and dissolved oxygen.