Plastic debris and seabird presence in the Hauraki Gulf,New Zealand

The potential threat of plastic pollution to seabirds within the Hauraki Gulf was assessed by determining its abundance and distribution at two different spatial scales and assessing the community of resident seabirds during July to September 2008. Samples of floating plastics within the gulf were taken during 66 trawls from the surface of the water in three regions within the inner gulf and Waitamata harbour. Within each of these regions, samples were taken in and outside of visually identified natural slick lines that are surface manifestations of small-scale convergence zones. During each trawl, the identity and abundance of seabirds were assessed. Plastic debris was recorded in all three regions sampled with the greatest abundance being recorded in the harbour. Plastics and debris were also significantly more concentrated within natural slick lines. Both user and industrial plastics were identified and consisted of colours and lengths likely to be mistaken as food items for small to medium seabirds. Surface or near-surface feeding seabirds in the gulf most likely mistakenly to consume plastics were fluttering shearwaters, Cape pigeons and Buller's shearwaters. However, these birds were most abundant in the area of the gulf where abundance of plastic pollution was lowest. The Hauraki Gulf is considered significantly less polluted by plastic debris than other severely polluted areas cited in literature.     

Thermohaline Anomalies in the Spring and Early Summer of 2000 in the Gulf of Trieste

Abstract. In the framework of the Interreg II Project (July 1998 - June 2001), hydro-logical, chemical and biological data were collected in the Gulf of Trieste.
During spring and summer 2000, some particular thermohaline anomalies were observed in the Gulf of Trieste. Especially in May and June the water body showed: a very strong thermohaline stratification, an increase of advective salt water coming from the south and the presence of sharp pycnoclines. In July the temperature was higher than usual in the whole water column. Moreover, in late May and in June, massive mucilaginous aggregates were observed along the water column and at the surface.
In order to highlight these particular thermohaline features the hydrological data of 16 stations were analysed (Fig. 1). Two stations, in particular, were considered: one offshore (St. AA1, average depth 20 m) and one close to the coast (St. C1, average depth 17 m). For these two stations a best-fit analysis, computed over 11 and 7 years, respectively, was performed on temperature, salinity and density excess data.
Moreover, the hydrological features were compared with the rainfall, air temperature, wind speed data (Istituto Sperimentale Talassografico di Trieste - ISTT) and the Isonzo River's flow rate (Direzione Regionale dell'Ambiente - Regione F. V. G.) collected from January 1998 to December 2000.     

The Ecology of Amorphous Aggregations (Marine Snow) in the Northern Adriatic Sea:

Abstract. In the water-column of the Gulf of Trieste (Northern Adriatic Sea) flocculent, amorphous aggregates known as marine snow reach maximum density and dimensions up to 20cm in diameter during summer months. Sampling of these fragile macro-aggregates was carried out by SCUBA-divers.
A large detrital component was found to be embedded in an organic matrix together with a large array of intact phytoplankton species. Heterotrophic bacterial density ranged from 2–65 × 10⁸ cells-g^-1 (marine snow dry wt) and is therefore of the same order of magnitude as in the surface layer of the sediments of the Gulf. In terms of biomass, bacteria reached only 30–95% of heterotrophic microflagellates.
A mean abundance of marine snow of 5g (marine snow dry wt) m^-3 was obtained for the water-column above the pyenolinc. At this boundary layer, however, a mean abundance of about 10g (marine snow dry wt) m^-3 led to the formation of a distinct, 10-50cm thick layer. Based on these data and sedimentation rates of marine snow to the pyenoclinc (38 g [marine snow dry wt] m^-2 d^-1) the potential ecological significance of these macro-aggregates for the pelagic system of the Gulf of Trieste is discussed.     

Analyses of the Relationship Between Mucous Aggregates and Phytoplankton Communities in the Gulf of Trieste (Northern Adriatic Sea) by Multivariate Techniques

Abstract. Multivariate techniques (cluster analysis and Principal Component Analysis) applied on phytoplankton data collected in June, July, August, and September 1990, 1991, 1992, and 1993 in the Gulf of Trieste (Northern Adriatic Sea) highlighted a clear temporal pattern from 1990 to 1993. In the summer of 1991, phytoplankton communities showed a dramatic increase in the dinoflagellatddiatom ratio. In particular, during June and July 1991 only few specimens of diatoms ( Cylindrorhecu closreriurn. Probosciu alata, Nirzschio longissimu , and Nitzschiu spp.) were collected; this confirmed the diatom scarcity in the phytoplankton communities before and during the appearance of large mucous aggregates (mucilage) in July 1991 in the Gulf of Trieste. The differences observed between the structure of the phytoplankton communities inside the aggregates and in the ambient waters suggested two hypotheses: 1) the aggregates were not produced in the gulf, but were carried ints the gulf by the eastern bordering current; 2) the aggregates were produced by a few, scarce species with a high exudate production that are capable of quickly reaching a high reproductive rate within the aggregates.     

Analysis of trophic networks and carbon flows in south-eastern Baltic coastal ecosystems

Carbon flows in five south-eastern Baltic coastal ecosystems (Puck Bay, Curonian Lagoon, Lithuanian coast, Gulf of Riga coast and Pärnu Bay) were compared on the basis of ECOPATH models using 12 common functional groups. The studied systems ranged from the hypertrophic Curonian Lagoon to the mesotrophic Gulf of Riga coast. Interestingly, we found that macrophytes were not consumed by grazers, but rather channelled into the detritus food chain. In all ecosystems fisheries had far reaching impacts on their target species and on the food-web in general. In particular, benthic food-webs were partly affected by indirect fisheries effects. For example, fisheries tend to change the biomass of piscivorous fish, causing a cascading effect on benthivorous fish and macrozoobenthos. These cascades are ecosystem specific and need to be considered when using benthic invertebrates as productivity and eutrophication indicators. Odum’s maturity attributes allowed a ranking of costal ecosystems according to their maturity. Namely, the community development decreased in the following order: Pärnu Bay > Gulf of Riga coast > Lithuanian coast > Puck Bay > Curonian Lagoon.     

The effects of entrainment on plankton at Koeberg nuclear power station

The plankton community entrained in seawater used for cooling at Koeberg Nuclear Power Station was studied to determine the effects of short-term exposure to heat, chlorine and physical stress on plankton survival and productivity. Field-based studies conducted between September 1985 and February 1988 revealed that phytoplankton biomass, measured as chlorophyll a, was reduced by an average of 55,33 per cent following entrainment, and average productivity was depressed by 38,29 per cent, mainly as a result of chlorination. Zooplankton mortality averaged 17,48 per cent for all species and 21,88 per cent for copepods, the dominant group. Plankton entrainment at Koeberg was not considered to be particularly detrimental to the marine environment because of the localized area affected, the rapid dispersion of heat and chlorine, the rapid regeneration times of phytoplankton and some zooplankton, the low abundance of commercially important species, and the potential for recruitment from the surrounding productive Benguela upwelling region. A review of entrainment effects at overseas power stations is incorporated in the discussion.     

The Epibiotic and Endolithic Species Associated with the Gastropod Shells inhabited by the Hermit Crabs Paguristes Oculatus and Pagurus Cuanensis

Abstract. Over 120 species, including epizoites, endolithic organisms, and an epiflora are associated with the gastropod shells inhabited by the hermit crabs Paguristes oculatus and Pagurus cuanensis in the North Adriatic Sea (Gulf of Trieste). The symbiotic organisms include members of three trophic levels - primary producers (rhodophytes), primary consumers (active and passive filter feeders), and secondary consumers (carnivorous species). Many of the invertebrates show a preferred distribution on the shells. The shells are a mobile substratum for a specific community. They are a sediment- and predator-free surface, and for their size are extremely stable substrates in terms of both time and orientation.
The hermit crab microbiocoenosis is a dynamic phenomenon that can be studied as a functional unit. The symbionts are responsible for two simultaneous processes - a constructive and a destructive process on the shells. Shell deterioration is due primarily to biological processes, not to mechanical destruction. The more frequently occurring species were classified according to their role in reinforcing or weakening shell structure. These developments are related to hermit crab activity. Epizoites grow through several shell changes by hermit crabs, and a number of forms are capable of surviving the final deposition of the shells.     

The Influence of Mucous Aggregates on the Microphytobenthic Community in the Northern Adriatic Sea

Abstract. An extended reappearance of mumus aggregates in the Northern Adriatic Sea in 1991 gave rise to the hypothesis of a possible influence of the mucus, settled on the bottom, on benthic flora and fauna.
This work investigates the variations in the microphytobenthic community in three stations of the Gulf of Trieste (AA1, AA2, AA3) during 1991, when mucous aggregates were present, and during the two following years, when no such aggregates were observed.
Water samples were collected by N iskin bottle in the bottom layer for nutrient analysis, and sediment samples were collected by divers. The diatom species and cell densities were determined under an inverted microscope. All the data were processed with the aim to evaluate community composition and possible relations with the presence of mucous aggregates in 1991.
The results showed a general decrease of benthic diatoms from 1991 to 1993, both in quality and quantity, for all the investigated sites. Statistical analyses on microphytobenthos, hydrological parameters, and nutrient concentrations indicated differences among the considered years. The microphytobenthic community in summer 1991 appeared to be quite different from those of the other years. The density of benthic diatoms was apparently not correlated with nutrient availability in the bottom layer, but was rather influenced by changes in temperature.
The presence of a dense microphytobenthic community in 1991 might be explained by a combination of adequate conditions related to the presence of mucous aggregates, including a more undisturbed substratum caused by the interruption of dredging, decreased grazing pressure due to a declined filtering capacity of epifauna, and nutrient-rich sediment for extra nutrient disposal remineralized at the mucus-sediment interface. The mucilage aggregates therefore apparently stimulated the microphytobenthic community, in contrast to the effects on the benthic macrofauna, which were perturbed by the mucus.     

Spatio‐temporal variability of zooplankton biomass and environmental control in the Northern Benguela Upwelling System: field investigations and model simulation

Spatial and temporal distribution patterns of zooplankton are highly variable in the Northern Benguela Upwelling System. We studied the distribution of zooplankton (size class ≥ 0.33 mm) and used field data from four cruises that took place between March 2008 and February 2011, as well as simulation results of a regional ecosystem model. Remotely sensed sea surface temperatures (SST) and surface chlorophyll concentrations were analysed to investigate environmental influences on zooplankton biomass. The Intense Benguela Upwelling Index showed a distinct seasonal signal throughout the years and the highest upwelling peaks in August/September. Even though surface chlorophyll concentrations were very variable throughout the year, the highest concentrations were always detected in September, following the upwelling of nutrient‐rich water. In field catches, zooplankton biomass concentration in the upper 200 m was highest above the outer shelf and shelf‐break in December 2010 and February 2011, i.e. 6 months after the upwelling peaks. In contrast, zooplankton biomass simulated by the model in the surface water was highest in September. In March/April, biomass maxima were typically measured in the field at intermediate water depths, but the vertical distribution was also affected by extensive oxygen minimum zones. The ecosystem model reproduced this vertical pattern. Although general trends were similar, simulation data of zooplankton standing stocks overestimated the field data by a factor of 3. In upwelling systems, food webs are generally considered to be short and dominated by large cells. However, our field data indicate more small‐sized zooplankton organisms above the shelf than offshore.     

Temporal variability of Centropages typicus in the Mediterranean Sea over seasonal-to-decadal scales

Centropages typicus is one of the most common, abundant and best studied calanoid copepods in neritic waters of the Mediterranean Sea, which means it can provide useful information about the long-term dynamics of the Mediterranean epipelagic ecosystem. This paper presents the first comparative overview of the seasonal and long-term variability of C. typicus in different Mediterranean regions. This review is based on quantitative information from the published literature and novel data from five ongoing zooplankton time-series carried out in the Mallorca Island (Balearic Sea), the Bay of Villefranche (Ligurian Sea), the Gulf of Naples (Tyrrhenian Sea), the Gulf of Trieste (North Adriatic Sea), and the Saronikos Gulf (Aegean Sea). In most Mediterranean regions, C. typicus has a perennial occurrence, with peaks of abundance that reflect the succession of different generations. Throughout the Mediterranean, the annual cycle of C. typicus is characterized by minima in winter and major peaks in April–June, which is earlier than those observed in European Atlantic waters, where the peaks are more frequently recorded in summer and fall. In the regions investigated, the annual cycle shows remarkable similarities in terms of timing, but notable differences in the peak height; populations are far more abundant in coastal north-western regions and less abundant in the eastern basin. In the long-term, changes in C. typicus phenology observed in the Bay of Villefranche and in the Gulf of Naples are related to the North Atlantic Oscillation (NAO) index. In these two regions, the species responds to climate forcing similarly in terms of average seasonal patterns (bi-modal patterns in years of positive NAO, unimodal patterns in years of negative NAO) but oppositely in terms of quantity, indicating different influence of the NAO on the two regions. At decadal scales, C. typicus populations show high interannual variability with marked geographical differences. In some areas, the patterns are clearly characterized by alternate phases of higher and lower annual abundances, at higher frequency (mainly 1–2 years) in the Gulf of Naples, and lower frequency in the Saronikos Gulf (mainly 4–5 years) and in the Gulf of Trieste (mainly 5–6 years). Synchronous phases of increasing or decreasing abundance are discernable only for a few sites and short periods, for example from 1998 to 2000 in the Gulf of Naples, Gulf of Trieste and Saronikos Gulf. The regional differences observed in the long-term patterns of C. typicus populations suggest that the temporal dynamics of this species are significantly more affected by local conditions than by any possible common driving force acting at basin scale through teleconnections.     

Depth-stratified community zonation patterns on Gulf of Alaska rocky shores

Vertical zonation patterns have been considered ubiquitous in intertidal ecosystems but questions remain about their generality for individual taxonomic groups and over broad spatial scales, and whether they continue into adjacent shallow subtidal habitats. Taxon richness, invertebrate abundance, and macroalgal biomass were examined in the summer of 2003 along a vertical gradient in the rocky intertidal and shallow subtidal habitats around Kodiak Island, Kachemak Bay, and Prince William Sound, all within the Gulf of Alaska. Replicate samples of benthic organisms were taken in the high (～ 7 m), mid (～ 4 m) and low (～ 0 m) intertidal (relative to MLLW), and at 1, 5, 10 and 15 m water depths at three sites in each region, and identified to the lowest possible taxonomic level. Our primary goals were to assess (1) how estimates of taxon richness, invertebrate abundance, and macroalgal biomass vary among intertidal heights and subtidal depths and (2) how general these patterns are when considered across the Gulf of Alaska. Our results show that when all invertebrates were considered together, most of the variation in taxon richness was accounted for by differences among depths (i.e. intertidal heights and subtidal depths) (～ 51%), and among replicate samples within each depth (～ 26%). Little to none of the variation was accounted for by differences among sites within each region (～ 1%) or among regions themselves (～ 0%). When considered across the Gulf of Alaska, total taxon richness and organism abundance were greatest in the low intertidal/shallow subtidal and decreased with increasing height/depth. When separated by phylum and examined together with macroalgae, variation in abundance and/or biomass among depths was significant and accounted for most of the variability. Differences among regions and sites within each region were not significant and accounted for little to none of the variance. Because the pattern of zonation varied among sites within each region, it reduced the generality of a single zonation pattern for the Gulf of Alaska. Likewise, when community composition was compared among depths, geographic regions and sites within each region using multivariate analyses, vertical zonation patterns were evident at a regional scale, but high variability in these patterns among sites within each region reduced the generality of these patterns.     

Factors affecting habitat occupancy of fish assemblage in the Gulf of Trieste (Northern Adriatic Sea)

Species composition, richness and abundance of the fish assemblage were studied over six different inshore macrohabitats in the southern part of the Gulf of Trieste (Northern Adriatic Sea) using the visual counts technique. Fifteen environmental variables were taken into consideration in order to determine microhabitat preferences of fish species, using canonical correspondence analysis and electivity indices. The results suggest that the structure of the fish assemblage in shallow habitats is affected by a large number of interplaying factors. Depth, type of bottom and vegetation cover – incorporating both abiotic and biotic variables – are some of the factors responsible for coastal fish distribution. Microhabitat preferences for 29 fish species are presented in the depth range from 0.5 to 3 m.     

Recent contamination of mercury in an estuarine environment (Marano lagoon,Northern Adriatic,Italy)

The Marano Lagoon, in the northern Adriatic Sea (Italy), has been affected by mercury (Hg) contamination from industrial and mining activities. It has been estimated that 186,000 kg of Hg were deliberately discharged into the main drainage system (Aussa–Corno River) by a chlor-alkali plant (CAP) from 1940s to 1984. The lagoon has also experienced a secondary long-term Hg input, originated from mining activity in Idrija (Slovenia), due to the supply of fluvial suspended particles carried by the Isonzo River in the Gulf of Trieste. Since local fishing activities are extensively conducted, there is great concern on the risk posed by potentially harmful effects of Hg to the trophic chain.     

Spatio-Temporal Distribution of Algal and Animal Communities in a Posidonia oceanica Meadow

Abstract. Investigations were carried out in a Posidonia oceanica meadow at Ischia (Gulf of Naples) along a depth gradient from 1 to 32 m in November 1979 and May 1981.
In these two months, different discontinuities were found at the sampling stations (1, 5, 10, 20, 30 m) at 10 m for leaf features, between 1 and 5 m for the algal community and the vagile fauna. The leaf features (length, surface, and biomass) along the transects show a delay in leaf production toward the deeper stations, below the thermocline.
The algal community shows in both seasons and in all stations a persistence of an encrusting layer, mostly represented by Corallinaceae and the brown alga Myrionema orbiculare , while at the shallowest station (1 m) the community is characterized by a more developed upright layer.
The vagile fauna of the leaf stratum, mostly represented by Polychaeta, Mollusca , and Crustacea (Peracarida and Decapoda) , shows a consistent zonation in both seasons along the transect. A superficial community at I m, characterized by a low number of species and individuals and specialized for high environmental energy levels, and a deeper community, persistent in time and more strictly related to the Posidonia oceanica meadows, are identified.
The influence of environmental factors and the importance of meadow structure for the zonation of the algal and animal communities are discussed.     

Microhabitat preferences and depth distribution of combtooth blennies (Blenniidae) in the Gulf of Trieste (North Adriatic Sea)

The microhabitat preferences and depth distribution of blenniid species (Blenniidae) in the Gulf of Trieste (North Adriatic Sea) were surveyed using the all‐occurrence sampling method, a non‐destructive visual census method, aided by SCUBA diving. Fourteen species were identified during the entire survey. Four species showed to be indiscriminate in their microhabitat choice while 10 species were classified as infrequent in the surveyed coastal area. A statistically significant correlation was found between the blenniid assemblage and nine microhabitat variables. The blenniid assemblage was divided in two main groups, by the use of canonical correspondence analysis, electivity index, and the depth distribution analysis. The first group comprises species that dwell in surface waters and show a high positive correlation with boulders, the presence of Mytilus galloprovincialis, cirripeds and empty holes bored by Lithophaga lithophaga. The second group includes species that mostly inhabit deeper waters and show a high positive correlation with rocks covered by precoralligenous bioformations.     

Region-wide impairment of Atlantic croaker testicular development and sperm production in the northern Gulf of Mexico hypoxic dead zone

Recently evidence has been obtained for reproductive impairment in estuarine populations of Atlantic croaker exposed to seasonal hypoxia. However, it is not known whether a similar disruption of reproductive function occurs in croaker inhabiting a much larger hypoxic area, the extensive dead zone in coastal regions of the northern Gulf of Mexico extending from the Mississippi Delta to East Texas. Gonadal development in male Atlantic croaker collected in September 2008 at six sites in the dead zone was compared to that in male fish sampled from three reference sites east of the Mississippi Delta which do not experience persistent hypoxia. Croaker testes collected from the dead zone were at an earlier stage of spermatogenesis than those from the reference sites. Histological examination of the testes collected from the dead zone showed that their tubules had small lumens that contained very little sperm compared to the lumens of the reference fish. Overall, sperm production was 26.2% that of the control fish at the reference sites. This decrease in spermatogenesis at the dead zone sites was accompanied by an approximately 50% decrease in testicular growth compared to that in the reference fish. The results suggest that reproductive impairment can occur over regional scales in marine fish populations exposed to extensive seasonal hypoxia in dead zones with potential long-term impacts on population abundance.     

Occurrence of copepod carcasses in the lower Chesapeake Bay and their decomposition by ambient microbes

We tested and refined the Neutral Red staining method for separating live and dead copepods in natural samples. Live copepods were stained red whereas dead copepods remained unstained. The staining results were not affected by method of killing, time of death or staining time. Tow duration had no significant effect on the percent dead copepods collected. The Neutral Red staining method was applied to study the occurrence of dead copepods along the York River and the Hampton River in the lower Chesapeake Bay during June–July, 2005. The zooplankton community was dominated by copepods; on average 29% of the copepod population appeared dead. Recovery of percent dead copepods did not differ between horizontal tows and vertical tows, suggesting that dead copepods were homogenously distributed in the water column. No significant relationship was found between the percent dead copepods and surface water temperature, salinity, Secchi depth or chlorophyll concentration. In laboratory experiments, dead copepods were decomposed by ambient bacteria and the rate of decomposition was temperature-dependent. Combining field and laboratory results we estimated that the non-consumptive mortality (mortality not due to predation) of copepods in the lower Chesapeake Bay was 0.12 d⁻¹ under steady-state condition, which is within the global average of copepod mortality rate.     

A numerical model investigation of the residual circulation in Hauraki Gulf,New Zealand

The homogeneous residual circulation in Hauraki Gulf arising from the tides, steady winds, and oceanic inflows is considered by use of a depth‐averaged 2‐dimensional numerical model. Vertical current structure of the wind‐driven circulation is derived by using the computed wind‐induced sea surface slopes, the wind stress, and a prescribed vertical eddy viscosity. Tidal residual circulation is weak, less than 0.01 ms^‐1 over most of the Gulf. The response of the Gulf to wind‐forcing indicates a preference for north‐west/south‐east directed winds, the flow through the Gulf being more than 3 times as strong as for winds from other directions. Surface currents are mainly in the wind direction, but subsurface currents reveal closed circulation cells in near‐coastal areas. Simple oceanic inflows give rise to water movements which penetrate to the inner part of the Gulf.     

Spatio‐temporal variations in demersal fish assemblages and diversity in the northern Gulf of St. Lawrence (Canada)

Species diversity is generally considered one of the key factors of ecosystem resilience in response to anthropogenic pressures, including fishing. In this context, the spatial and temporal changes in demersal fish assemblages and species diversity were investigated in the northern Gulf of St. Lawrence (Canada), over a 20‐year period (1990–2010). Data were obtained from the summer research survey conducted by the Department of Fisheries and Oceans, and include commercial and non‐commercial species. The study covers the period of groundfish fishery collapse, the moratorium period, and the post‐moratorium period, and reflects various modifications in management. Multivariate statistical methods revealed two communities. A coastal community corresponds to strata located above 200 m depth and a deeper community located in the deep channels. Interannual differences in the composition of fish assemblages were observed and are mainly due to the changes in the relative biomass of some dominant species. Three diversity indices (Shannon–Wiener, Simpson's Index of Diversity and Motomura's constant) indicate a slight but significant increase of the diversity over time. This trend is due to the increase of the relative biomass of low‐rank species, which may have been favoured by the prohibition of groundfish trawling after 1997 in that region. The geographical distribution of the Shannon–Wiener index also shows temporal dynamics reflecting the biomass distribution of dominant demersal species.     

Influences of initial plankton biomass and mixed-layer depths on the outcome of iron-fertilization experiments

Several in situ iron-enrichment experiments have been conducted, where the response of the phytoplankton community differed. We use a marine ecosystem model to investigate the effect of iron on phytoplankton in response to different initial plankton conditions and mixed-layer depths (MLDs). Sensitivity analysis of the model results to the MLDs reveals that the modeled response to the same iron enhancement treatment differed dramatically according to the different MLDs. The magnitude of the iron-induced biogeochemical responses in the surface water, such as maximum chlorophyll, is inversely correlated with MLD, as observed. The significant decrease in maximum surface chlorophyll with MLD results from the difference in diatom concentration in the mixed layer, which is determined by vertical mixing. The modeled column-integrated chlorophyll, on the other hand, is the highest with intermediate MLD cases, suggesting difference in iron-induced biogeochemical responses between volume and area considerations. The iron-induced diatom bloom is severely restricted below the compensation depth due to both light limitation and grazing pressure, irrespective of the MLD. Sensitivity of the model to initial mesozooplankton (as grazers on diatoms) biomass shows that column-integrated biomass, net community production and export production are strongly controlled by the initial mesozooplankton biomass. Higher initial mesozooplankton biomass yields high grazing pressure on diatoms, which results in less accumulation of diatom biomass and may account for notably lower surface chlorophyll during SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) II than during SEEDS. The initial diatom biomass is also important to the outcome of iron enrichment but is not as crucial as the MLD and the initial mesozooplankton biomass. This modeling study suggests that not only MLD but also the initial biomass of diatoms and its principle grazers are crucial factors in the response of the phytoplankton community to iron enrichments, and should be considered in designing future iron-enrichment experiments.