Biological Characteristics of Central Indian Basin Waters During the Southern Summer

Phytoplankton biomass, taxonomy, primary productivity, and photosynthetically available radiation (PAR) were studied as part of baseline data collection for prospective nodule mining in the Central Indian Basin during the ORV Sagar Kanya cruise SK-120 in January 1997. The phytoplankton cell counts and chlorophyll a estimates showed low biomass level, suggesting low rates of primary productivity in the region studied. The average chlorophyll a value was 0.775 mg m^?3 at surface and 17.75 mg m^?2 in the water column. Similarly, average primary productivity at surface was 3.72 mg C m^?3 d^?1 and was 51.23 mg C m^?2 d^?1 in column. The chlorophyll a maxima at 50 to 80 m was the characteristic feature of the euphotic zones of the area. Average phytoplankton counts at the surface were low (3960 cells/l), compared to those at 25 m (6421 cells/l) and 75 m (5187 cells/l). At most of the stations mesozooplankton biomass was maximum in the top 50 m water column, indicating the importance of grazing in the euphotic zone. Appreciable quantities of mesozooplankton were observed below the euphotic zone, where settlement of chlorophyll a occurs. The low iron concentration in the water and its relationship with the water column productivity were correlated. The results show that waters in the CIB have low productivity in the surface as well as subsurface layers. This is expected to change in this case of a mining discharge in to these layers, possibly locally affecting the existing marine ecosystems. The final impact of such mining activity may remain negligible in the deep sea environment.     

Impacts of the wintertime mesozooplankton community to downward carbon flux in the subarctic and subtropical Pacific Oceans

We compared wintertime depth distributions of the mesozooplankton community and dominant copepods between the subtropical (S1) and subarctic (K2) Pacific Oceans to evaluate the relative importance of actively transported carbon by vertical migrants to sinking particulate organic carbon flux. Primary production was higher and the ratio of sinking particulate organic carbon flux to primary production was lower at S1 compared with those at K2. The mesozooplankton community was lower in abundance and biomass at S1 compared to K2. Copepods were the dominant group among both mesozooplankton abundance and biomass throughout the water column down to 1000 m at both sites. The depth distribution showed that diel vertical migration was obvious for the mesozooplankton abundance and biomass at S1 but was not apparent for the abundance at K2, because the dominant component was diurnally migrating species at S1 and overwintering species residing at mesopelagic depths at K2. The major components of diel migrants were copepods and euphausiids at S1 and only euphausiids at K2. Respiratory flux by the diurnally migrating mesozooplankton was estimated to be 2 mgC m⁻² day⁻¹ at S1 and 7 mgC m⁻² day⁻¹ at K2. The respiratory flux was equivalent to 131% of sedimentary fecal pellet flux at S1 and 136% of that at K2. Because pathways of downward carbon flux are facilitated by the mesozooplankton community, the actively transported carbon (respiration of dissolved inorganic carbon, excretion of dissolved organic carbon and egestion of fecal pellets at depth) might be larger during winter than the flux of sinking fecal pellets.     

Summer mesozooplankton structure in the Pechora Sea (south-eastern Barents Sea)

Mesozooplankton composition and distribution were investigated by Juday net hauls in the Pechora Sea (south-eastern Barents Sea) in July 2001. A total of 66 taxa were identified. The total mesozooplankton abundance varied between 2416 ind m⁻² in the northern part and 1458?935 ind m⁻² in the south. The biomass ranged between 81 and 19?078 mg DW m⁻². Three groups differed greatly in composition, abundance and biomass as delineated by cluster analysis. Copepod species Calanus finmarchicus, Pseudocalanus species and Limnocalanus macrurus dominated in terms of the total biomass within each single cluster. There were significant Spearman rank correlations between mesozooplankton abundance and oceanographic variables, and phytoplankton concentration. Salinity was the main factor affecting the mesozooplankton distribution in the coastal waters, while temperature had greater influence on the abundance and biomass in the central and northern parts. The mean mesozooplankton biomass in the region was higher in comparison with some previous investigations probably due to higher water temperature in summer 2001.     

北太平洋亚热带环流区与西太平洋暖池区浮游动物群落垂直分布的差异及其对碳通量的意义

The mesozooplankton in both epipelagic and mesopelagic zones is essentially important for the study of ecosystem and biological carbon pump. Previous studies showed that the diel vertical migration(DVM) pattern of mesozooplankton varied among ecosystems. However, that pattern was largely unknown in the Western Pacific Warm Pool(WPWP). The vertical distribution, DVM and community structure of mesozooplankton from the surface to 1 000 m were compared at Stas JL7K(WPWP) and MA(North Pacific Subtropical Gyre, NPSG). Two sites showed similarly low biomass in both epipelagic and mesopelagic zones, which were in accordance with oligotrophic conditions of these two ecosystems. Stronger DVM(night/day ratio) was found at JL7K(1.31) than that at MA(1.09) on surface 0–100 m, and an obvious night increase of mesopelagic biomass was observed at JL7K, which was probably due to migrators from bathypelagic zone. Active carbon flux by DVM of zooplankton was estimated to be 0.23 mmol/(m2·d) at JL7K and 0.16 mmol/(m~2·d) at MA. The community structure analysis showed that calanoid copepods, cnidarians and appendicularians were the main contributors to DVM of mesozooplankton at both sites. We also compared the present result with previous studies of the two ecosystems,and suggested that the DVM of mesozooplankton was more homogeneous within the WPWP and more variable within the NPSG, though both ecosystems showed typically extremely oligotrophic conditions. The different diel vertical migration strength of mesozooplankton between NPSG and WPWP implied different efficiency of carbon pump in these two ecosystems.     

Mesozooplankton in the Arctic Ocean in summer

The biomass, species and chemical composition of the mesozooplankton and their impact on lower food levels were estimated along a transect across the Arctic Ocean. Mesozooplankton biomass in the upper 200 m of the water column was significantly higher (19–42 mg DW m^-3) than has previously been reported for the Arctic Ocean, and it reached a maximum at ca. 87°N in the Amundsen Basin. The lowest values were recorded in the Chukchi Sea and Nansen Basin, where ice cover was lower (50–80%) than in the central Arctic Ocean. In the deeper strata (200–500 m) of the Canadian and Eurasian Basins, the biomass was always much lower (4.35–16.44 mg DW m^-3). The C/N (g/g) ratio for the mesozooplankton population was high (6.5–8.5) but within the documented range. These high values (when compared to 4.5 at lower latitudes) may be explained by the high lipid content. Mesozooplankton accounted for approximately 40% of the total particulate organic carbon in the upper 100 m of the water column. Mesozooplankton species composition was homogeneous along the transect, consisting mainly of copepods (70–90% of the total number). It was dominated by four large copepod species (Calanus hyperboreus, C. glacialis, C. finmarchicus and Metridia longa), which together accounted for more than 80% of the total biomass. According to measurements of gut pigment and gut turnover rates, the mesozooplankton on average ingested between 6 and 30% of their body carbon per day as phytoplankton. Microzooplankton may have provided an additional source of energy for the mesozooplankton community. These data emphasize the importance of mesozooplankton in the arctic food web and reinforce the idea that the Arctic Ocean should no longer be considered to be a “biological desert”.     

Mesozooplankton biomass and grazing responses to Cyclone Opal, a subtropical mesoscale eddy

As part of E-Flux III cruise studies in March 2005, plankton net collections were made to assess the effects of a cyclonic cold-core eddy (Cyclone Opal) on the biomass and grazing of mesozooplankton. Mesozooplankton biomass in the central region of Cyclone Opal, an area of uplifted nutricline and a subsurface diatom bloom, averaged 0.80±0.24 and 1.51±0.59 g DW m⁻², for day and night tows, respectively. These biomass estimates were about 80% higher than control (OUT) stations, with increases more or less proportionately distributed among size classes from 0.2 to >5 mm. Though elevated relative to surrounding waters south of the Hawaiian Islands (Hawai’i lee), total biomass and size distribution in Cyclone Opal were almost exactly the same as contemporary measurements made at Stn. ALOHA, 100 km north of the islands, by the HOT (Hawaii Ocean Time-series) Program. Mesozooplankton biomass and community composition at the OUT stations were also similar to ALOHA values from 1994 to 1996, preceding a recent decadal increase. These comparisons may therefore provide insight into production characteristics or biomass gradients associated with decadal changes at Stn. ALOHA. Gut fluorescence estimates were higher in Opal than in ambient waters, translating to grazing impacts of 0.11±0.02 d⁻¹ (IN) versus 0.03±0.01 d⁻¹ (OUT). Over the depth-integrated euphotic zone, mesozooplankton accounted for 30% of the combined grazing losses of phytoplankton to micro- and meso-herbivores in Opal, as compared to 13% at control stations. Estimates of active export flux by migrating zooplankton averaged 0.81 mmol C m⁻² d⁻¹ in Cyclone Opal and 0.37 mmol C m⁻² d⁻¹ at OUT stations, 53% and 24%, respectively, of the carbon export measured by passive sediment traps. Migrants also exported 0.18 mmol N m⁻² d⁻¹ (117% of trap N flux) in Cyclone Opal compared to 0.08 mmol N m⁻² d⁻¹ (51% of trap flux) at control stations. Overall, the food-web importance of mesozooplankton increased in Cyclone Opal both in absolute and relative terms. Diel migrants provided evidence for enhanced export flux in the eddy that was missed by sediment trap and ²³⁴Th techniques, and migrant-mediated flux was the major export term in the observed bloom-perturbation response and N mass balance of the eddy.     

Seasonal horizontal and vertical variability in primary production and standing stocks of phytoplankton and zooplankton in the Cretan Sea and the Straits of the Cretan Arc (March 1994–January 1995)

Phytoplankton communities, production rates and chlorophyll levels, together with zooplankton communities and biomass, were studied in relation to the hydrological properties in the euphotic zone (upper 100 m) in the Cretan Sea and the Straits of the Cretan Arc. The data were collected during four seasonal cruises undertaken from March 1994 to January 1995.The area studied is characterised by low nutrient concentrations, low ¹⁴C fixation rates, and impoverished phytoplankton and zooplankton standing stocks. Seasonal fluctuations in phytoplankton densities, chlorophyll standing stock and phytoplankton production are significant; maxima occur in spring and winter and minima in summer and autumn. Zooplankton also shows a clear seasonal pattern, with highest abundances occurring in autumn–winter, and smallest populations in spring–summer. During summer and early autumn, the phytoplankton distribution is determined by the vertical structure of the water column.Concentrations of all nutrients are very low in the surface waters, but increase at the deep chlorophyll maximum (DCM) layer, which ranges in depth from about 75–100 m. Chlorophyll-a concentrations in the DCM vary from 0.22–0.49 mg m⁻³, whilst the surface values range from 0.03–0.06 mg m⁻³. Maxima of phytoplankton, in terms of cell populations, are also encountered at average depths of 50–75 m, and do not always coincide with chlorophyll maxima. Primary production peaks usually occur within the upper layers of the euphotic zone.There is a seasonal succession of phytoplankton and zooplankton species. Diatoms and ‘others’ (comprising mainly cryptophytes and rhodophytes) dominate in winter and spring and are replaced by dinoflagellates in summer and coccolithophores in autumn. Copepods always dominate the mesozooplankton assemblages, contributing approximately 70% of total mesozooplankton abundance, and chaetognaths are the second most abundant group.     

Downward export of carbon by diel migrant mesozooplankton in the central equatorial Pacific

Using data collected during cruises of the JGOFS equatorial Pacific Study in March/April and October of 1992 at the equator (140°W), we examine the downward transport of carbon by three size classes of die] migrant mesozooplankton (200–500 gm, 500–1000 μm and 1000–2000 gm). In addition to respiratory carbon flux, we consider the flux due to mortality of migrators below the euphotic zone. Diel migrant mesozooplankton biomass was estimated from the difference between nighttime and daytime biomass within the euphotic zone. Except for a four-day period early in the March/April cruise, mesozooplankton nighttime biomass was significantly larger than daytime biomass within the euphotic zone during both cruises. We estimate that the downward flux of carbon from the euphotic zone due to mesozooplankton die] vertical migrators was an average of 0.6 mmol Cm⁻² d⁻¹ and 1.1 mmol C m⁻² d⁻¹ during the March/April and October cruises, respectively. Addition of this flux to the gravitational particle sinking flux estimated from²³⁴Th measurements during the same period results in a 31 % increase in the carbon export flux from the euphotic zone in the equatorial Pacific during the March/April cruise and a 44% increase in the October cruise. The migratory flux is strongly dependent on whether feeding takes place below the euphoric zone, the length of time migrators spend in the deep waters, and the mortality rate of migrators.     

Plankton community diversity from bacteria to copepods in bloom and non-bloom conditions in the Celtic Sea in spring

The plankton community composition comprising heterotrophic bacteria, pro-/eukaryotes, heterotrophic nanoflagellates, microzooplankton and mesozooplankton was assessed during the spring bloom and at non-bloom stations in the English Channel and Celtic Sea between 6 and 12 April 2002. Non-bloom sites were characterised by a dominance of pro-/eukaryotic phytoplankton <20 μm, higher abundance of heterotrophic nanoflagellates, microzooplankton standing stocks ranging between 60 and 380 mg C m⁻², lower mesozooplankton diversity and copepod abundance of between 760 and 2600 ind m⁻³. Within the bloom, the phytoplankton community was typically dominated by larger cells with low abundance of pro-/eukaryotes. Heterotrophic nanoflagellate cell bio-volume decreased leading to a reduction in biomass whereas microzooplankton biomass increased (360–1500 mg C m⁻²) due to an increase in cell bio-volume and copepod abundance ranged between 1400 and 3800 ind m⁻³. Mesozooplankton diversity increased with an increase in productivity. Relationships between the plankton community and environmental data were examined using multivariate statistics and these highlighted significant differences in the abiotic variables, the pro-/eukaryotic phytoplankton communities, heterotrophic nanoflagellate, microzooplankton and total zooplankton communities between the bloom and non-bloom sites. The variables which best described variation in the microzooplankton community were temperature and silicate. The spatial variation in zooplankton diversity was best explained by temperature. This study provides an insight into the changes that occur between trophic levels within the plankton in response to the spring bloom in this area.     

Seasonal variability in carbon demand and flux by mesozooplankton communities at subarctic and subtropical sites in the western North Pacific Ocean

We investigated seasonal changes in carbon demand and flux by mesozooplankton communities at subtropical (S1) and subarctic sites (K2) in the western North Pacific Ocean to compare the impact of mesozooplankton communities on the carbon budget in surface and mesopelagic layers. Fecal pellet fluxes were one order higher at K2 than at S1, and seemed to be enhanced by copepod and euphausiid egestion under high chlorophyll a concentrations. The decrease in pellet volume and the lack of any substantial change in shape composition during sink suggest a decline in fecal pellet flux due to coprorhexy and coprophagy. While respiratory and excretory carbon by diel migrants at depth (i.e., active carbon flux) was similar between the two sites, the actively transported carbon exceeded sinking fecal pellets at S1. Mesozooplankton carbon demand in surface and mesopelagic layers was higher at K2 than S1, and an excess of demand to primary production and sinking POC flux was found during some seasons at K2. We propose that this demand was met by supplementary carbon sources such as feeding on protozoans and fecal pellets at the surface and carnivory of migrants at mesopelagic depths.     

Biomass, Abundance, and Vertical Distribution of Micronekton and Large Gelatinous Zooplankton in the Subarctic Pacific and the Bering Sea during the Summer of 1997

The biomass, abundance, and vertical distribution of micronekton, including enidarians, mysids, euphausiids, decapods, thaliaceans, and fishes, were studied on the basis of samples collected with an 8-m² opening-closing rectangular midwater trawl (RMT-8, mesh size: 4.5 mm) at three stations in the subarctic Pacific (the western subarctic gyre, the central Subarctic, and the Gulf of Alaska) and one station in the oceanic Bering Sea. The total biomass in the 0–1000 m water column ranged from 2.9 to 5.1 gDW m^–2. Except for primary consumers that showed highly variable biomass (thaliaceans and euphausiids), biomass was highest in the oceanic Bering Sea followed by the central (boundary between eastern and western gyres), western gyre, and eastern Gulf of Alaska. The biomass compositions by higher taxa were basically similar between regions: fishes were most dominant, followed by enidarians at all stations, except for the marked predominance of thaliaceans in the Gulf of Alaska. High biomasses of gelatinous animals (31% of overall dry weight), occasionally comparable to those of fishes and crustaceans, suggest their potential importance in the subarctic Pacific. Characteristics in vertical patterns of micronekton biomass common in all stations were: (1) a mesopelagic peak around 500–600 m both day and night, (2) a layer of low biomass in the cold intermediate water and/or in the upper mesopelagic zone, (3) a nighttime shift of biomass to upper layers, and (4) an highly variable biomass of epipelagic/interzonal migrants (euphausiids and thaliaceans).     

Biomass and trophic structure of the plankton community in subtropical and temperate waters of the northwestern Pacific Ocean

This study examined the biomass structure of autotrophic and heterotrophic plankton along a trophic gradient in the northwestern Pacific Ocean in an attempt to understand planktonic food web structure. Autotrophic biomass exceeded that of heterotrophic organisms in all sampling regions, but with lesser contribution to total planktonic biomass at stations of higher phytoplankton biomass, including the northern East China Sea, compared to the regions of lower phytoplankton biomass. The proportion of the biomass of heterotrophic bacteria, nanoflagellates (HNF), and dinoflagellates (HDF) relative to that of phytoplankton was all inversely related to phytoplankton biomass, but positive relationships were observed for both ciliates and mesozooplankton. Mesozooplankton biomass inclined greater than phytoplankton along the gradient of phytoplankton biomass, with biomass rise being most closely associated with ciliate and HDF biomass and, to a lesser degree, with large phytoplankton (>3?μm). Both bacteria and picophytoplankton were significantly and positively related to the biomass ratio of mesozooplankton to the sum of HDF and ciliates (i.e., proxy of mesozooplankton predation on protozoans), but no positive relationship was apparent either for HNF or for large phytoplankton. Such relationships may result from predation relief on lower food webs associated with mesozooplankton feeding on protistan plankton.     

Biological organic carbon export estimated from the annual carbon budget observed in the surface waters of the western subarctic and subtropical North Pacific Ocean from 2004 to 2013

The annual flux of biologically produced organic carbon from surface waters is equivalent to annual net community production (NCP) at a steady state and equals the export of particulate and dissolved organic carbon (POC and DOC, respectively) to the ocean interior. NCP was estimated from carbon budgets of salinity-normalized dissolved inorganic carbon (nDIC) inventories at two time-series stations in the western subarctic (K2) and subtropical (S1) North Pacific Ocean. By using quasi-monthly biogeochemical observations from 2004 to 2013, monthly mean nDIC inventories were integrated from the surface to the annual maximum mixed layer depth and corrected for changes due to net air–sea CO₂ exchange, net CaCO₃ production, vertical diffusion from the upper thermocline, and horizontal advection. The annual organic carbon flux at K2 (1.49 ± 0.42 mol m^?2 year^?1) was lower than S1 (2.81 ± 0.53 mol m^?2 year^?1) (p < 0.001 based on t test). These fluxes consist of three components: vertically exported POC fluxes (K2: 1.43 mol m^?2 year^?1; S1: 2.49 mol m^?2 year^?1), vertical diffusive DOC fluxes (K2: 0.03 mol m^?2 year^?1; S1: 0.25 mol m^?2 year^?1), and suspended POC fluxes (K2: 0.03 mol m^?2 year^?1; S1: 0.07 mol m^?2 year^?1). The estimated POC export flux at K2 was comparable to the sum of the POC flux observed with drifting sediment traps and active carbon flux exported by migrating zooplankton. The export fluxes at both stations were higher than those reported at other time-series sites (ALOHA, the Bermuda Atlantic Time-series Study, and Ocean Station Papa).     

Anchovy spawning in relation to the biomass and the replenishment rate of their copepod prey on the western Agulhas Bank

Anchovy biomass and copepod standing stocks and growth rates on the Agulhas Bank were compared during the peak spawning period (November) in 1988 and 1989. In 1988, copepod biomass over the western Agulhas Bank was low (1,0 g dry mass·m^?2) relative to anchovy biomass there (14,7 g dry mass·m^?2). In November 1989 in the same area, fish biomass was much lower (5,7 g dry mass·m^?2), following a recruitment failure, and copepod biomass was higher (2,4 g dry mass·m^?2), possibly as a result of lesser predation by anchovy. By contrast, the eastern Agulhas Bank had a larger biomass of copepods (4–6 g dry mass·^?2) and a lower biomass of anchovy during both years. Knowing, from laboratory studies, that a prey biomass of 0,78 g·m^?2 is required for fish to obtain their daily maintenance ration, it is suggested that spawning on the western Agulhas Bank was food-limited in 1988. Copepods on the western Bank may be replaced by local growth or transport from the eastern Bank. Growth rates of copepods on the western Bank were 10–50 per cent of maximum in 1988, but total production (c. 100 mg dry mass·m^?2·day^?1) was low, primarily because biomass was low and less than the rate of consumption by anchovy (243 mg copepod dry mass·m^?2·day^?1). On the eastern Bank, copepod production exceeded anchovy consumption and it is concluded that the flux of copepod biomass onto the western Bank may be as important as local growth in replenishing copepod stocks there. Feeding conditions for anchovy on the western Agulhas Bank are often marginal compared to the situation on the eastern Bank, and it is suggested that the selection of the western Bank as the major spawning area is related more to the success of transport and survival of eggs and larvae on the West Coast recruiting grounds than to feeding conditions per se.     

Zooplankton distribution patterns at two seamounts in the subtropical and tropical NE Atlantic

Spatial distribution patterns of zooplankton biomass in relation to local and large‐scale hydrographical and biological driving forces were studied at Ampère and Senghor, two shallow seamounts in the subtropical and tropical NE Atlantic, respectively. The study includes a first assessment of the taxonomic composition and an estimation of the respiratory carbon demand of the zooplankton community. Zooplankton was sampled during three cruises at the seamount and open ocean reference sites in May and October 2009 and in December 2010. Zooplankton standing stocks and the corresponding respiratory carbon demand were about six times higher at Senghor than at Ampère, with mean stocks of 24.7 and 4.6 g·m^?2, respectively, in the upper 1000 m. Mean respiratory carbon demand in the epipelagic zone was calculated as 61.4 mg·C·m^?2·day^?1 for Senghor and 9.6 mg·C·m^?2·day^?1 for Ampère. At neither site were differences between seamount and open ocean sites significant. However, horizontal surveys across Ampère Seamount show clear differences between day and night distributions and a reduced biomass above the summit. Across Senghor, zooplankton biomass increased from the SW to the NE flank, with the highest concentrations in the subsurface layer of the chlorophyll maximum and just above a strong oxycline. The zooplankton community at Ampère Seamount reflects the oligotrophic character of the NE Atlantic subtropical gyre, whereas the nutrient‐rich waters of the cyclonic tropical gyre at Senghor support a higher biomass. This difference in the zooplankton biomass between the two seamounts can be attributed to the large‐scale hydrographical features governing the productivity regimes rather than to regional seamount effects.     

Spatio‐temporal dynamics and biomass of Cymodocea nodosa in Bekalta (Tunisia,Southern Mediterranean Sea)

Leaf growth, biomass and production of Cymodocea nodosa were measured from October 2006 to September 2007 in Monastir Bay (Tunisia). Shoot density showed a clear seasonal pattern, increasing during spring and summer and decreasing during fall and winter. Monthly mean shoot density ranged between 633 ± 48 and 704 ± 48 shoots?m^?2. The monthly average total biomass ranged between 560 ± 37 and 646 ± 32 g dry weight (DW)?m^?2. Total biomass varied significantly among stations and sampling times but did not show seasonal variation. Leaf plastochrone intervals varied seasonally, with an annual average of 28–30 days. Leaf productivity was highest in August (2.61 g DW?m^?2?day^?1) and lowest in February (0.35 g DW?m^?2?day^?1). Annual belowground primary production varied from 263 to 311 g DW?m^?2?year^?1. Annual leaf production was approximately equal for all the stations (from 264 to 289 g DW?m^?2?year^?1). Variability in water temperature, air temperature and salinity explained the annual variability in biological characteristics. Changes in belowground and total biomass were not correlated with seasonal variability in the environmental parameters monitored. Additionally, a literature review was conducted of C. nodosa features at other Mediterranean sites, encompassing 30 studies from 1985 to 2014.     

Distribution of zooplankton in the Barents Sea in August 2006

Forty-two mesozooplankton samples were collected in the Barents Sea during the cruise of the R/V Dal’nie Zelentsy in August 2006. In total, 72 taxa of planktic animals were found. The minimal average mesozooplankton abundance was noted in the Murmansk coastal waters in the south of the sea (154 ind./m³), while the maximal was noted in the Arctic waters (1533 ind./m³). The average wet biomass varied from 32 to 830 mg/m³. The zooplankton abundance and biomass exceeded the relevant average long-term parameters by 1.5–2 times and by 1.2–1.4 times, respectively. The mean biodiversity (Shannon’s index) of the zooplankton communities was low: H′ = 1.62 ± 0.104 bit/ind.     

Land use,associated eel production,and abundance of fish and crayfish in streams in Waikato,New Zealand

Abstract

The density and biomass of fish and crayfish, and the production of eels, was compared among streams in native forest, exotic forest, and pasture. Populations were estimated by multiple‐pass electroshocking at 11 sites in hill‐country streams in the Waikato region, North Island. Three sites were in native forest, four in exotic forest, and four in pasture. Length of stream sampled at each site was 46–94 m (41–246 m² in area), and catchment areas up stream of the sites ranged from 0.44 to 2.01 km².

A total of 487 fish were caught. The species were longfinned and shortfinned eels, banded kokopu, Cran's and redfinned bullies, and common smelt. Eels were the most abundant fish in all three land‐use types, and shortfinned eels were more abundant at pastoral sites (mean density 1.11 fish m^?2) than longfinned eels (mean density 0.129 fish m^?2). Banded kokopu were present only at forested sites. Mean fish densities were greater at pastoral sites (1.55 fish m^?2) than under either native forest (0.130 fish m^?2) or exotic forest (0.229 fish m^?2). Mean fish biomass was also greater at pastoral sites (89.7 g m^?2) than under native forest (12.8 g m^?2) or exotic forest (19.3 g m^?2). Longfinned eels made a greater contribution to the fish biomass at all sites than did shortfinned eels. Densities of crayfish were high (0.46–5.40 crayfish m^?2), but were not significantly different between land‐use types. Crayfish biomass ranged from 1.79 to 11.2 g m^?2. Total eel production was greater at pastoral sites (mean 17.9 g m^?2 year¹) than at forest sites (mean 2.39 gm^?2 year^?1).     

Estimates of phytoplankton and bacterial biomass and production in the northern and southern Benguela ecosystems

Available data on phytoplankton and bacterial abundance and production off the coasts of southern Africa (to the 500 m depth contour) have been assembled and analysed for a network analysis of carbon flow in the Benguela ecosystem. Phytoplankton carbon biomass (from measurements of chlorophyll a) in the northern Benguela (2 558 300 tons) was considerably higher than in the southern Benguela (671 420 and 516 400 tons for the West and South coasts respectively). However, overall annual production (from C¹⁴-uptake measurements) was similar, 77 416 608, 76 399 973 and 78 988 020 tons C·year^?1 respectively. Phytoplankton respiration and sedimentation losses were calculated as functions of primary production and therefore followed similar trends. From the most conservative estimates (mean bacterial biomass of 10 mg C·m^?3 and average P:B of 0,2·day^?1) bacterial biomass is 2–7 per cent of phytoplankton biomass in the northern and southern Benguela, and bacterial production is 3–5 per cent of primary production. Assuming a net growth yield of 30 per cent, bacteria would need to consume 9–15 per cent of the total primary production in order to meet their requirements for carbon consumption. Calculations based on a mean bacterial biomass of 40 mg C·m^?3 and a mean growth rate of 0,5·day^?1 in the upper 30 m of the water column show bacterial biomass to be 8–27 per cent of phytoplankton biomass and bacterial production to be 26–44 per cent of phytoplankton production. Bacterial carbon consumption requirements at these rates amount to 86–147 per cent of total primary production.