Relative importance of benthic microalgae,phytoplankton, and mangroves as sources of nutrition for penaeid prawns and other coastal invertebrates from Malaysia

Carbon, sulfur, and nitrogen stable isotope ratio techniques were used in 1989 and 1990 to evaluate the relative importance of algae and of mangrove detritus in the nutrition of two penaeid prawn species on the west coast of Peninsular Malaysia. Mangrove detritus was found to contribute to the nutrition of juvenile Penaeus merguiensis de Man living within tidal creeks, but not to adult P. merguiensis and juvenile and adult Parapenaeopsis sculptilis (Heller) captured offshore. Results from radiotracer feeding studies, with refractory ¹⁴C mangrove lignocellulose as the food source, indicated that juvenile P. merguiensis from tidal creeks assimilated mangrove carbon with an efficiency of 13.4%. This did not differ significantly (P=0.05) from the assimilation efficiencies of juvenile and adult P. sculptilis living offshore (10.0 and 10.9%, respectively); these values were significantly higher (P<0.001) than for adult P. merguiensis (2.1%). Thus, the differential contribution of mangrove material to the nutrition of prawns in the tidal creeks and offshore was not related to differences in the prawn's ability to utilize detritus. Instead, our analysis of C stable isotopes in sediments indicated that mangrove detritus was generally more abundant within the tidal creeks than offshore. Juvenile prawns in the tidal creeks may also utilize mangrove material indirectly by feeding on small detritivorous invertebrates. Stable isotope analyses suggest that benthic microalgae constitute the other major dietary component for prawns living in tidal creeks. Prawns offshore were utilizing mainly phytoplankton-based material.     

Geographical variations in the trophic ecology of Japanese anchovy, Engraulis japonicus, inferred from carbon and nitrogen stable isotope ratios

Carbon and nitrogen stable isotope ratios of Japanese anchovy (Engraulis japonicus) and their stomach contents were examined and compared among various regions around Japan. Geographical variations in the isotope ratios were found between inshore and Pacific offshore regions. While most of the anchovy samples had isotope ratios around −17.6‰ for δ¹³C and 10.0‰ for δ¹⁵N as median values, higher (more enriched) isotope values were found in the anchovy sampled from inshore regions. On the contrary, lower (more depleted) values were found mostly in the anchovy from the Pacific offshore region including the Kuroshio Extension and Kuroshio-Oyashio transition zones. Higher carbon isotope ratios in the inshore regions may reflect a carbon source from benthic primary producers in addition to phytoplankton possibly through the consumption of the larvae of benthic organisms such as bivalves or decapods, which were found in the stomach contents of the inshore anchovy. Variations in the nitrogen isotope ratio may reflect not only differences in the trophic level of prey species, but also variations in the baseline level of food webs. Stable isotope ratios are potentially a useful tool for understanding the stock/population structure and migration of anchovy. The present findings indicate the potential importance of the “inshore–offshore” variations in the biology of Japanese anchovy populations in the northwestern Pacific waters.     

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000     

Effects of temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis

To assess the effects of both temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), individuals collected December 1996 (winter experiment) and June 1997 (summer experiment) were maintained for 3 months in one of four experimental treatments: (1) 3 °C and fed ad libitum (high ration), (2) 3 °C and fed one-seventh of the maximum ration (low ration), and (3) 12 °C and fed the high ration; (4) 12 °C and fed the low ration. All individuals were fed an artificial diet and exposed to only 1 h of light every day. At the end of both experiments, mean gonad indices of sea urchins fed the high ration had increased significantly (11–24% and 6–19% in the winter and summer experiments, respectively), while the gonad indices of individuals fed the low ration did not change. At the high ration (both experiments), the increase in gonad index of sea urchins occurred primarily as the result of a significant increase in the mass of nutritive phagocytes, as revealed by histological analyses. Primary oocytes were significantly larger in individuals held at 3 °C than at 12 °C throughout the winter experiment, regardless of food ration; during the summer experiment, primary oocytes were significantly larger in individuals receiving the high ration, regardless of the temperature at which they were held. These results suggest that: (1) food availability is the most important factor regulating energy storage and the relative size of gonads throughout the year, (2) temperature affects the rate of growth and maturation of primary oocytes during the later stages of oogenesis, and (3) once gametogenesis has been initiated, mature ova will be produced, even under conditions of low food availability. Conditions of high food availability in summer and low temperature in winter would thus favor reproductive output in sea urchin populations. Received: 1 March 2000 / Accepted: 4 October 2000     

Temporal and spatial variability in stable isotope ratios of SPM link to local hydrography and longer term SPM averages suggest heavy dependence of mussels on nearshore production

Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ¹³C and δ¹⁵ N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration.     

Role of grapsid crabs, Parasesarma erythrodactyla, in entry of mangrove leaves into an estuarine food web: a mesocosm study

The influence of the crab Parasesarma erythrodactyla on the entry of the organic matter derived from Avicennia marina mangrove leaves in a sub-tropical mangrove ecosystem of southeast Queensland, Australia, was simulated using tidal mesocosms. Degradation of mangrove leaf organic matter was followed by analysing the fatty acid composition, carbon, and nitrogen isotopic signatures of the surface sediment and suspended particulate organic matter (SPOM) with and without the presence of crabs. Assimilation of mangrove organic matter by P. erythrodactyla was also assessed by stable isotope and fatty acid analyses in tissues and faeces. Results of the chemical tracer analyses question the adaptability of P. erythrodactyla to a diet comprised exclusively of mangrove leaves, and suggest that these organisms were dependent on additional food sources in their natural environment. Crab processing of senescent leaves significantly accelerated the transfer of mangrove organic matter to the surface sediments, as shown by a higher C/N ratio, a higher contribution of long-chain fatty acids and a more depleted C isotopic signature of sediment samples in the mesocosms with crabs compared to those without crabs.     

Effects of ambient ammonia levels on blood ammonia, ammonia excretion and heart and scaphognathite rates of Nephrops norvegicus

During commercial handling of Nephropsnorvegicus (L.) there are a number of situations when the prawns may be exposed to very high ambient ammonia levels. These experiments evaluated the effects of increased levels of ambient total ammonia (TA = NH₃ + NH₄ ⁺) on␣blood ammonia, ammonia efflux rates and on the cardio-ventilatory performance of N. norvegicus. When prawns were taken from <1 to 2000 μmol TA l⁻¹ medium, blood TA concentrations increased rapidly for the first 2 h but tended to drop thereafter. Original blood TA levels were restored 6 h after the prawns were transferred back from seawater containing 2000 to <1 μmol TA l⁻¹. Sudden exposure to 500, 1000, 2000 or 4000 μmol TA l⁻¹ medium induced blood TA concentrations to increase respectively to 50, 30, 33 and 36% of external concentrations (normally, internal TA values are much higher than external levels). Immediately after transfer back to seawater with low ammonia concentration ( <1 μmol TA l⁻¹), excretion rates were higher than those of control prawns, and the absolute amounts of TA excreted were considerably higher than those calculated to have accumulated in the haemolymph. Heart rate (HR) and scaphognathite rate (SR) were not altered when prawns were subjected to sudden alterations in ambient ammonia ( <1 to 2000 to <1 μmol TA l⁻¹). When water ammonia concentrations were altered more gradually, both rates increased, but only at 4000 μmol TA l⁻¹. These results show that N. norvegicus is able to remove ammonia from the haemolymph and/or transform ammonia into some other substance when subjected to increased levels of ambient ammonia. Possible mechanisms involved (e.g. active transport across the gills; storage in some other tissue; glutamate synthe sis) are discussed. Received: 20 May 1996 / Accepted: 1 July 1996     

Competition and interaction between Neosarmatium smithi (Crustacea: Grapsidae) and Terebralia palustris (Mollusca: Gastropoda) in a Kenyan mangrove

 In a Kenyan mangrove, we studied the interactions between the gastropod Terebralia palustris and the crab Neosarmatium smithi when foraging on decaying mangrove leaves. Interactions are considerable on account of their wide overlap in zonation (Rhizophora mucronata belt), food items (mangrove leaves) and activity window (diurnal low water). The snails find a leaf by a systematic transecting of the platform and eat the leaf after crawling on to it, usually en masse. The crabs rarely venture beyond 80 cm from their burrows, and once they find a leaf they quickly drag it back into their burrow, probably to reduce the strong intra-specific competition. A crab is able to drag a leaf away even if a number of snails are already feeding on it, by strongly pulling the leaf away or by pushing the snails off the leaf. The success of the “thief ” depends on both the crab's size and the number of snails on the leaf. Received: 25 July 1999 / Accepted: 27 April 2000     

Mercury in a northeastern Brazilian mangrove area, a case study: potential of the mangrove oyster Crassostrea rhizophorae as bioindicator for mercury

The Channel of Santa Cruz is a mangrove area in northeastern Brasil 40 km north of Recife. Until 1991 a chlor-alkali factory discharged large amounts of mercury into the main tributary of the channel, the Rio Botafogo. The objective of this study was to assess the potential of the mangrove oyster Crassostrea rhizophorae as a bioindicator for mercury in this area and to examine the influence of condition on mercury concentrations in the oysters. The investigation was carried out in the late rainy season (July to September) 1993, in the rainy season (April to June) 1994 and in the dry season (November/December) 1994. At 11 stations in the channel, mangrove oysters, surface sediments and suspended matter were sampled, representing different compartments of the system. The distribution of mercury in the Channel of Santa Cruz exhibited for all compartments the same distinct spatial pattern with maxima in the Rio Botafogo. Seasonal variations were small and revealed no clear tendencies. In addition, mangrove oysters were transplanted from more contaminated stations to less contaminated stations and vice versa. These experiments were designed to study the oysters' capability to adjust their mercury concentrations to a changing bioavailability of mercury. The transplantation experiments suggest that the oysters are capable to adjust to changing ambient mercury availability within a few months. The transplantation experiments also revealed the importance of condition changes on the mercury concentration of the oysters. This factor should not be ignored during pollution studies. However, condition had apparently no strong influence on the spatial or seasonal mercury variations of the oysters. The mangrove oyster is a suitable bioindicator for long-term changes in mercury availability. Received: 20 August 1997 / Accepted: 15 January 1998     

Diurnal and lunar patterns of larval recruitment of Brachyura into a mangrove estuary system in Ranong Province, Thailand

The present study followed the temporal recruitment pattern of brachyuran larvae in a mangrove tidal creek on the Andaman Sea coast of Ranong Province, Thailand, based on the assumption that the processes governing recruitment are important for the overall population dynamics of mangrove brachyuran crabs. Plankton net samples were taken on five occasions: on two new moon spring tides, one waxing moon neap tide, one full moon spring tide and one waning moon neap tide during October and November 1997. In addition collectors for larval crab megalopae were employed every 3 days through one dry season and one wet season (March–October 1998). Both the plankton net samples and collector samples revealed four major brachyuran groups in three families: Ocypodidae, Grapsidae and Portunidae. The grapsid group was further separated into two morphotypes which were identified as Metaplax and sesarmid species. Identified group mean numbers per cubic metre were ocypodids 3.0, sesarmids 0.8 and Metaplax 0.5, while portunid megalopae were very scarce (≪0.1 m⁻³). Further analysis of plankton net samples showed that when considering the parameters date, depth, current direction and the diel cycle, Metaplax and ocypodids distribute differently in the tidal and lunar cycle. Metaplax recruitment dominates on flood tides and on bottom layers, followed by middle and surface layers. Conversely, ocypodid abundance varied significantly with date only. Notably recruitment was not dependent on the diel cycle for either group. The collector samples of megalopae showed that recruitment of ocypodids, Metaplax and sesarmids occurred on full and new moon spring tides, while portunid megalopae preferred to settle on full moon spring tides. Since tidal currents were related to the lunar cycle megalopa groups are also cross-correlated with tidal amplitude, except for the portunid group. It is concluded that megalopae recruit in a similar manner to what has been found in other regions of the world, except that the abundance of ocypodids and Metaplax is not influenced by the diel cycle. Received: 14 February 2000 / Accepted: 24 November 2000     

Nitrate storage by phytoplankton in a coastal upwelling environment

The storage of nitrate by phytoplankton cells during the early phases of upwelling was studied in coastal stations off northern Spain (southern Bay of Biscay) between 1990 and 1994. In this region, a persistent upwelling during summer is characterised by intermittent pulses of variable intensity, and increased nutrient concentrations in the surface layer. The main effect of an upwelling pulse on phytoplankton distribution is the shifting of the chlorophyll a and primary production maxima to near the surface. When the upwelling relaxes, thermal stratification of the water column occurs, and a distinct subsurface chlorophyll maximum develops below the production maximum. An accumulation of intracellular nitrate characterized the early phases of upwelling (mean = 2.73 μmol N m⁻³), maximum concentrations being attained at depths where biomass and production values were moderate. In contrast, phytoplankton cells from non-upwelling situations contained significantly lower concentrations of intracellular nitrate (mean = 0.17 μmol N m⁻³). The variations in the intracellular pool of nitrate may result from the differential allocation of resources within the cell as a result of variations in the energy available, since the uptake and assimilation of nitrate is a relatively expensive process involving several enzymatic systems. We hypothesize that nitrate storage by phytoplankton cells is characteristic of early phases of upwelling and is linked to patterns of carbon fixation. Average nitrogen budgets for upwelling and non-upwelling situations indicate that intracellular nitrate reserves are not responsible for maintaining high phytoplankton growth rates, since they only account for <2% of daily primary production during upwelling events. Received: 28 August 1996 / Accepted 3 December 1996     

Growth and development rates of Calanus finmarchicus nauplii during a diatom spring bloom

Growth and development rates were determined for nauplii of Calanus finmarchicus (Gunnerus) in the near-shore waters of a western Norwegian fjord from in situ mesocosm incubations. The major food source for the nauplii was diatoms, but Phaeocystis sp., dinoflagellates and ciliates were also part of the diet. At local temperatures ranging from 4.8 to 5.2 °C the cumulative median development time from hatching to Nauplius VI was 19 d. The time taken to molt to the next naupliar stage was approximately constant (3 d) from Stages IV to VI, but Stage III needed the longest development time (5 d). The instantaneous growth rate in terms of body carbon was negative from hatching to Nauplius Stage II, but as high as 0.25 to 0.30 d⁻¹ from Stage III to V. Enhancement of food resources by nutrient addition led to no significant change in specific growth rates. Additionally, the cohorts from different nutrient regimes showed almost equal development time, size and body carbon within stages. Length–weight relationships of nauplii from the two different food resources were: W _{low resources} = 4.17 × 10⁻⁶ × L ^2.03 (r ² = 0.84) and W _{high resources} = 4.29 × 10⁻⁶ × L ^2.05 (r ² = 0.92), where weight (W) is in micrograms of C and body length (L) in micrometers. The natural body morphology of naupliar stages I to VI is illustrated with digital images, including the final molt from Nauplius VI to Copepodid Stage I. In general, development of the nauplii was faster than that of the copepodids of C. finmarchicus, and structural growth was exponential from naupliar stages III to VI. This study validates our earlier results that nauplii of C. finmarchicus can obtain high growth and nearly maximal developmental rates at relatively low food levels (∼50 μg C l⁻¹), suggesting that nauplii exhibit far less dependence on food supply than copepodids. Received: 30 July 1999 / Accepted: 7 March 2000     

Kelp as a trophic resource for marine suspension feeders: a review of isotope-based evidence

Kelp forests are enormously productive, and they and adjacent habitats support large populations of suspension feeders. What do these suspension feeders eat? Intuitively, we might expect that kelp primary production is a key form of trophic support for these animals. Indeed, a large and growing number of studies using carbon stable isotope data, typically collected over short time periods, have asserted that detritus from kelps is an important, and in some cases the main, food source for coastal benthic suspension feeders. This view has been incorporated into several textbooks and review papers covering kelp forest ecosystems, and loss of trophic support for benthic suspension feeders is now often invoked as an ecosystem consequence of top-down or other impacts on kelp forests. More direct evidence, however, suggests that these animals mainly eat phytoplankton and, in some cases, bacteria or zooplankton. Because isotope values of pure coastal phytoplankton, uncontaminated with detritus, are difficult to obtain, present studies have largely relied on single measurements from offshore environments or from the literature, which typically reflects offshore values. We review the evidence showing that phytoplankton isotope values can, and are expected to, vary widely in coastal waters and that inshore phytoplankton may often be enriched in ¹³C compared to offshore phytoplankton. This unaccounted-for variation may have systematically biased the results of such trophic studies toward finding large contributions of kelp detritus to suspension-feeder diets. We review some key stable isotope studies and put forth evidence for alternative explanations of the isotope patterns presented. Finally, we make recommendations for future isotope studies and describe several approaches for progress in this area. New techniques, particularly flow cytometry and compound-specific stable isotope analysis, provide ways to shed light on this interesting and important ecological issue.     

Inter-specific and geographical variations in the fatty acid composition of mangrove leaves: implications for using fatty acids as a taxonomic tool and tracers of organic matter

Fatty acid compositions of the leaves of six species of mangroves were studied to ascertain their use as biomarkers for determining the fate of mangrove organic matter in the habitat and as taxonomic tool. Mangrove leaves were collected from three locations in the western Pacific Ocean: Moreton Bay (MB) (Australia), Hong Kong (China) and Okinawa (Japan). In MB, samples were collected from two sites separated by 15 km: Logan River Estuary (LRE) and Jabiru Island. In addition, along the LRE, leaves were collected from five stations at ∼2–3 km apart. Results show that the analysis of the entire fatty acid profiles of the mangrove leaves is a promising taxonomic tool as the profiles of most species were sufficiently different to be separated in an non-metric multidimensional scaling plot. In addition, geographically separated populations of the same species could also be identified by their fatty acid profiles. In most cases, two non-ubiquitous groups of fatty acids dominated in the mangrove leaves: the polyunsaturated 18:2ω6 and 18:3ω3 and the long chain fatty acids (≥24:0). With respect to the relative contributions of these fatty acids, three groups of species were identified, in which one or both groups of fatty acids may potentially be used as markers of the mangrove organic matter in the estuarine environment.     

Zooplankton community structure across an eddy-generated upwelling band close to a tropical bay-mangrove ecosystem

Mesoscale eddies in the world’s oceans are ubiquitous and bring about episodic pulses of nutrients into the photic zone. Transient in nature, the role of eddy pumping in coastal enrichment via plankton production, and subsequent organic flux is not yet fully realised. In the context of a cyclonic cold-core eddy that propagates annually under the influence of the East India Coastal Current and enriches coastal waters in the western Bay of Bengal north of 16°N, a detailed study on zooplankton community structure along with phytoplankton composition and associated water quality was undertaken during April–May 2002 coinciding with the spring intermonsoon. Zooplankton samples were collected at 32 hydrographically different (salinity 24.5–35.6 PSU) GPS fixed locations representing bay-mangrove areas and nearshore waters (30 m) close to the River Godavari, which is one of India’s largest estuarine systems. During the study, the bay-coastal waters were typified by elevated nutrient levels (nitrate 10.73–22.04 μM), high salinity (27.98–35.52 PSU), and relatively low temperatures (30.63–31.40°C). Altogether, 95 zooplankton taxa were encountered with copepods forming the predominant population. Agglomerative Hierarchical Cluster Analysis (AHCA) and Non-metric Multidimensional Scaling (NMDS) based on Bray–Curtis similarity (PRIMER) analysis revealed appreciable alterations in zooplankton structure across bay-mangrove locations and coastal waters (Stress 0.11; ANOSIM test Global R: 0.94, P = 0.1%). Similarity Percentage (SIMPER) analysis revealed zooplankton associations through “discriminating species” for each location (Kakinada Bay, Cluster I, 27.9 ± 3.0 PSU; upwelling band, cluster II, 35.5 ± 0.2 PSU; offshore waters, cluster III, 34.2 ± 0.4 PSU; mangrove outlets, cluster IV, 32.7 ± 1.3 PSU and mangrove creeks, cluster V, 33.5 ± 0.6 PSU). The index of multivariate dispersion (IMD) illustrated high variability in zooplankton standing stock for mangrove/sea locations relative to the bay. Concurrent observations on phytoplankton revealed the importance of diatoms (r: 0.640, P ≤ 0.05). Within the eddy-generated band of upwelled water, a significant top-down control of diatoms by herbivorous zooplankton resulted in a comparative increase in abundance of dinophyceans. Based on zooplankton abundance data and species association patterns, it was possible to distinguish different zooplankton/copepod communities in accordance with mesoscale variability in physical, chemical and biological processes under tropical conditions. This was confirmed through canonical correspondence analysis (CCA) that represented coastal-offshore waters and the Bay environment in this area.     

Short-term variation and long-term changes in the oceanographic environment and zooplankton community in the vicinity of a sub-Antarctic archipelago

 Mesozooplankton community structure in the vicinity of the Prince Edward Islands (PEIs) was investigated during six surveys conducted in late austral summer (April/May) from 1996 to 1999. Zooplankton samples were collected by oblique tows using a Bongo net fitted with 300-μm mesh. Surface temperature, average temperature and chlorophyll a were measured in conjunction with each net tow. The positions of the Sub-Antarctic Front (SAF) and the Antarctic Polar Front (APF), in relation to the islands, were determined by CTD and/or XBT transects to the west of the islands (upstream). Both fronts were characterized by a high degree of latitudinal variation. Changes in position of the fronts occurred rapidly, the SAF moving up to ∼120 km in a 2-week period. Consequently, the oceanographic environment in the vicinity of the PEIs was subject to a high degree of intra- and inter-survey variation. The positions of the SAF and APF appeared to have a significant impact on phytoplankton biomass in the vicinity of the PEIs, possibly through the alteration of local oceanographic flow dynamics. Water retention over the island shelf in 1996, associated with location of the SAF far to the north of the PEIs, corresponded to enhanced chlorophyll-a concentrations (∼1.54 mg m⁻³). Conversely, when the fronts were close to the islands, as in 1997 and 1999, higher current velocity limited water retention and chlorophyll-a concentrations in the inter-island region were relatively low (∼0.4 mg m⁻³). Cluster analyses showed that, in many instances, there was greater similarity among zooplankton communities from different surveys than among communities within surveys, indicating that short-term variability exceeded inter-annual variability. The population structure of the copepod Calanus simillimus indicated that there was inter-annual variation in the timing of the biological season. Differences in the population structure of species, and consequently their contribution to abundance and biomass, may therefore have been an important contributor to inter-annual variation in community structure. Evidence is provided of a long-term southward shift in the position of the SAF. It is postulated that this may affect the PEIs by increasing the proportion of allochthonous energy input, because the PEIs now lie in the path of the front, altering the tropho-dynamics of the island ecosystem. Lower mesozooplankton biomass associated with warmer sub-Antarctic water may have important negative consequences for higher trophic levels that depend on mesozooplankton for food. Received: 10 June 2000 / Accepted: 22 September 2000     

Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

Phytoplankton production patterns in Massachusetts Bay and the absence of the 1998 winter–spring bloom

The seasonal productivity cycle and factors controlling annual variation in the timing and magnitude of the winter–spring bloom were examined for several locations (range: 42°20.35′–42°26.63′N; 70°44.19′–70°56.52′W) in Boston Harbor and Massachusetts Bay, USA, from 1995 to 1999, and compared with earlier published data (1992–1994). Primary productivity (mg C m⁻² day⁻¹) in Massachusetts Bay from 1995 to 1999 was generally characterized by a well-developed winter–spring bloom of several weeks duration, high but variable production during the summer, and a prominent fall bloom. The bulk of production (mg C m⁻³ day⁻¹) typically occurred in the upper 15 m of the water column. At a nearby Boston Harbor station a gradual pattern of increasing areal production from winter through summer was more typical, with the bulk of production restricted to the upper 5 m. Annual productivity in Massachusetts Bay and Boston Harbor ranged from a low of 160 g C m⁻² year⁻¹ to a high of 787 g C m⁻² year⁻¹ from 1992 to 1999. Mean annual productivity was higher (mean=525 g C m⁻² year⁻¹) and more variable near the harbor entrance than in western Massachusetts Bay. At the harbor station productivity varied more than 3.5-fold (CV=40%) over an 8 year sampling period. Average annual productivity (305–419 g C m⁻² year⁻¹) and variability around the means (CV=25–27%) were lower at both the outer nearfield and central nearfield regions of Massachusetts Bay. Annual productivity in 1998 was unusually low at all three sites (<220 g C m⁻² year⁻¹) due to the absence of a winter–spring phytoplankton bloom. Potential factors influencing the occurrence of a spring bloom were investigated. Incident irradiance during the winter–spring period was not significantly different (P > 0.05) among years (1995–1999). The mean photic depth during the bloom period was significantly deeper (P < 0.05) in 1998, signifying greater light availability with depth. Nutrients were also in abundance during the winter–spring of 1998 with stratified conditions not observed until May. In general, the magnitude of the winter–spring bloom in Massachusetts Bay from 1995 to 1999 was significantly correlated with winter water temperature (r ²=0.78) and zooplankton abundance (r ²=0.74) over the bloom period (typically February–April). The absence of the 1998 bloom was associated with higher than average water temperature and elevated levels of zooplankton abundance just prior to, and during, the peak winter–spring bloom period. Received: 3 July 2000 / Accepted: 6 December 2000     

Food utilisation of pelagic mysids, Mysis mixta and M. relicta, during their growing season in the northern Baltic Sea

The dietary habits of the pelagic mysid Mysismixta were studied during its growing season at an open sea location in the Gulf of Finland, the northern Baltic Sea. Stomach samples were taken twice a month from June to September 1997. The most abundant phytoplankton taxa in the stomachs were diatoms and dinoflagellates, and copepods and cladocerans were the most abundant zooplankton identified. A clear change was found in the diets during the study period. Small mysids (3 to 6 mm) fed on sedimented phytoplankton in the early summer (90% benthic particles in June) but shifted gradually to a more pelagic and carnivorous diet (>40% pelagic particles, consisting of ca. 60% zooplankton in September). Seasonal changes in mysid capture ability as well as food availability were suggested to affect the diet composition of mysids during their growth. The ratio of pelagic and benthic food particles could – irrespective of the season – be explained by mysid size, whereas the zooplankton:phytoplankton ratio was better explained by season. The stomach analysis suggests that the mysids needed to attain a threshold size of 8 to 11 mm to initiate feeding on the more evasive copepods. Mysids also started to grow faster at the same time as the proportion of copepods increased in the diet, which suggests that copepods are an important energy source for M. mixta in late summer. Finally, a comparison was made between the M. mixta diet and that of the less abundant M. relicta. The diets of the two pelagic mysid species overlapped by 75% (Schoener's index). The main difference was due to M. mixta eating more zooplankton and pelagic material than M. relicta. Received: 15 September 1999 / Accepted: 18 January 2000     

Inter-colony movements,at-sea behaviour and foraging in an immature seabird: results from GPS-PPT tracking,radio-tracking and stable isotope analysis

Seabird populations contain large numbers of immatures––in some instances comprising >50% of the fully grown adults in the population. These birds are significant components of marine food webs and may contribute to compensatory recruitment and dispersal, but remain severely understudied. Here, we use GPS-PTTs, radio-tracking and analysis of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to investigate the movements and foraging ecology of immature seabirds. Our study focussed on immature northern gannets Morus bassanus aged 2–4 attending non-breeding aggregations alongside a large breeding colony. GPS-PTT tracking of five birds revealed that immatures have the ability to disperse widely during the breeding season, with some individuals potentially prospecting at other colonies. Overall, however, immatures were faithful to the colony of capture. During returns to the focal colony, immatures acted as central place foragers, conducted looping and commuting flights, and analysis of the variance in first-passage time revealed evidence of area-restricted search (ARS) behaviour. In addition, stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses indicate that immatures were isotopically segregated from breeders. Our findings provide insights into the foraging, prospecting and dispersal behaviour of immature seabirds, which may have important implications for understanding seabird ecology and conservation.