Leif Pihl

Hypoxia-induced structural changes in the diet of bottom-feeding fish and Crustacea

Interactive effects of three alternating normoxia-hypoxia cycles on benthic prey exploitation by mobile fish (spot, Leiostomus xanthurus; and hogchoker, Trinectes maculatus) and a burrowing crustacean (Squilla empusa) were investigated in the York River, Chesapeake Bay, Virginia, USA, in 1989. Predators collected in four depth strata (A: 5 to 10 m; B: 10 to 14 m; C: 14 to 20 m; D:>20 m) variously affected by hypoxia were separated into size classes (three for spot and two each for hogchoker and mantis shrimp) to examine potential ontogenetic influences in prey selection. The most severe effects of hypoxia on the benthos occurred in the two deepest strata (C and D) and decreased in shallower strata (B>A), with Stratum A never affected by low oxygen. Predators investigated exhibited dietary evidence of optimal prey exploitation during or immediately after hypoxic events. In most instances gut contents contained significantly larger, deeper-burrowing prey during periods of low oxygen than during alternating peroids of normal oxygen levels. Spot consumed a greater biomass (45 to 73%) of polychaetes than other prey, with crustaceans initially also constituting a main dietary component but decreasing in importance later in the study period. The deep-burrowing anemone, Edwardsia elegans, was an important prey species for spot, particularly in the lower depth strata affected by hypoxia. Prey consumed by 10-to 15-cm-long spot increased significantly in size during some hypoxic events, suggesting a sublethal effect of hypoxia on large benthic species. Polychaetes (primarily Glycera americana, Notomastis latericeus and Loimia medusa) were dominant dietary components in hogchoker, making up between 85 and 98% of the diet. Bivalve siphons became important prey for hogchoker in the three deepest strata and were only consumed after the August hypoxia. Stomach contents of mantis shrimp were difficult to identify in most instances due to the near complete mastication of consumed prey. Crustaceans were important prey initially but became less conspicuous in the diet subsequent to the July hypoxia event, when hydroids became more dominant. Overall, predator species exhibited optimal exploitation of moribund or slowly recovering benthos affected by hypoxia. The sublethal effects of hypoxia through increased availability of benthos to resident predators can have important consequences for energy flow in areas such as the York River which experience periodic low-oxygen cycles.

Distribution of green algal mats throughout shallow soft bottoms of the Swedish Skagerrak archipelago in relation to nutrient sources and wave exposure

Abstract Distribution and biomass of green algal mats were studied in marine shallow (0–1 m) soft-bottom areas on the Swedish west coast from 1994 to 1996, by combining aerial photography surveys with ground truth sampling. Filamentous green algae, dominated by species of the genera Cladophora and Enteromorpha, were generally present throughout the study area during July and August, and largely absent in late April and early May. These algae occurred at 60 to 90% of the locations investigated during the summer, and were estimated to cover between 30 and 50% of the total area of shallow soft bottoms of the Swedish Skagerrak archipelago. The distributional patterns were similar during the three years of the investigation and appeared unrelated to annual local nutrient inputs from point sources and river discharge. We postulate that the apparent lack of such a relationship is due to an altered state of nutrient dynamics throughout the archipelago. Mechanisms are likely to involve long-term, diffuse elevations in nutrient levels in coastal waters of the Skagerrak and the Kattegat over several decades leading to current eutrophic conditions, exceeding nutrient requirements for abundant filamentous algal growth. Patterns of algal abundance in our study were largely related to physical factors such as exposure to wind, waves and water exchange under conditions where nutrient loads among embayments seemed to be unlimited. Further, our results show that sediments covered by algal mats had higher carbon and nitrogen contents than unvegetated sediments. We hypothesise that sustained high nutrient loads, manifested in extensive biomass of filamentous algae during summer months, are re-mineralised via decay and sedimentation in the benthic realm. Hence, accumulated carbon and nutrients in the sediment could, in turn, constitute the basic pool for future algal mat production overlying soft bottoms in areas where tidal exchange is limited.

Cannibal-prey dynamics in young juveniles and postlarvae of the blue crab

Although cannibalism can act as a density-dependent regulator of population size in terrestrial systems, little is known of its effects in the marine environment. Herein we investigate the influence of cannibalism upon the early life history stages of the blue crab, Callinectes sapidus Rathbun, emphasizing cannibalism between juveniles and postlarvae (i.e. megalopae) of the same year class. In laboratory mesocosms we examined various factors modulating cannibal–prey dynamics, specifically: (1) the effects of habitat and presence of conspecifics on postlarval metamorphosis rate; (2) the effect of metamorphosis rate on the mortality of postlarvae from both intra- and inter-cohort cannibalism; (3) the effects of habitat and predator density on the functional response of young juvenile blue crab predators to varying densities of postlarval prey, and (4) the effects of prey size and habitat on predation mortality. Inter-cohort cannibalism caused significant mortality in every crab size and habitat type combination, and was lower in grass than sand for all prey smaller than fifth instar. Cannibalism between postlarvae was associated with metamorphosis and was density-dependent in sand, but not present in grass. Metamorphosis rates of postlarvae were inversely density-dependent in sand, but density-independent and higher in grass, indicating that habitat and intra-cohort agonism likely affects postlarval metamorphosis rates. Inter-cohort cannibalism was negatively correlated with metamorphosis rates of postlarvae. The functional response of young juvenile cannibalistic blue crabs differed significantly between sand and grass habitats, and between medium and high predator densities. Juvenile crabs displayed a type II, inversely density-dependent functional response in sand, resulting in very high mortality at low densities of postlarval prey. In grass, the crabs displayed a weak type III, density dependent response, yielding significantly lower mortality at low prey densities. Thus, habitat complexity changes the form of the functional response in cannibal–prey interactions and grass provides a relative habitat refuge from cannibalism. Doubling the number of predators in grass decreased the consumption rates per predator significantly and eliminated the density-dependence, indicating that intraspecific density can qualitatively change the form of the functional response. In the crab size experiment, only prey smaller than fifth instars received a habitat refuge from cannibalism in grass, whereas fifth instars received a relative size refuge in sand. Our results demonstrate that intra-year class cannibalism can cause mortality upon settling megalopae and first juvenile instars that is dependent on prey density. We expect inter-cohort cannibalism to cause local extinction of cohorts settling in sand, especially at low settlement densities, and high mortality at moderate settlement densities in grass. Satiation of predators at high settlement densities in grass suggests that episodic settlement can overwhelm predators locally. Furthermore, density-dependent mutual interference within large cohorts in the grass beds likely reduces their predation efficiency, indicating that aggregation of conspecific predators in grass habitats does not necessary lead to an increase in predation pressure. Finally, a relative size-refuge from inter-cohort cannibalism for fifth instar crabs supports an ontogenetic habitat shift around this crab size, which may be influenced by density-dependent agonistic behavior within cohorts. We suggest that intra-year class cannibalism is a major process regulating both survival and dispersal in megalopae and juvenile blue crabs.

Ecosystem goods and services from Swedish coastal habitats: identification, valuation, and implications of ecosystem shifts.

Abstract Coastal areas are exposed to a variety of threats due to high population densities and rapid economic development. How will this affect human welfare and our dependence on natures capacity to provide ecosystem goods and services? This paper is original in evaluating this concern for major habitats (macroalgae, seagrasses, blue mussel beds, and unvegetated soft bottoms) in a temperate coastal setting. More than 40 categories of goods and services are classified into provisional, regulating, and cultural services. A wide variety of Swedish examples is described for each category, including accounts of economic values and the relative importance of different habitats. For example, distinguishing characteristics would be the exceptional importance of blue mussels for mitigation of eutrophication, sandy soft bottoms for recreational uses, and seagrasses and macroalgae for fisheries production and control of wave and current energy. Net changes in the provision of goods and services are evaluated for three cases of observed coastal ecosystem shifts: i) seagrass beds into unvegetated substrate; ii) unvegetated shallow soft bottoms into filamentous algal mat dominance; and iii) macroalgae into mussel beds on hard substrate. The results are discussed in a management context including accounts of biodiversity, interconnectedness of ecosystems, and potential of economic valuation.

Vanishing seagrass (Zostera marina, L.) in Swedish coastal waters.

Abstract Along the Swedish Skagerrak coast eelgrass (Zostera marina) is a dominant phanerogam on shallow soft bottoms. Eelgrass meadows are important biotopes for many crustacean and fish species being either migratory or stationary. During the 1980s, inventories of the shallow coastal areas with eelgrass have been carried out along the Swedish west coast as a basis for coastal zone management. In the present study we revisited 2000 ha of eelgrass meadows in 5 coastal regions along 200 km of the Skagerrak coast. The inventory was made with the same methods (aquascope) as during the 1980s, but increasing the mapping accuracy by using a Global Positioning System (GPS). The results from this study show that the areal extension of Zostera marina has decreased 58% in 10–15 years with great regional variations. The decline was mainly restricted to the shallow parts of the meadow. The causes and ecological consequences are discussed.

Responses to hypoxia of plaice, Pleuronectes platessa, and dab, Limanda limanda, in the south-east Kattegat: distribution and growth

SynopsisPlaice, Pleuronectes platessa, and dab, Limanda limanda, were sampled with a Glommen lobster trawl at 25 to 40 m depth in the SE Kattegat during spring and autumn of 1984 to 1990. During autumn, hypoxia (O2-concentration < 3 mg I−1) occurred in the bottom water below the halocline for four to ten weeks every year, except in 1984 when moderate hypoxia (O2-concentration 3–5 mg 1−1) occurred. Biomass of both species was shown to be negatively correlated with oxygen concentration during autumn. Further, a decrease in population mean total length was observed during the study period in both spring and autumn samples. Laboratory studies of growth of juvenile plaice and dab, at 15° C and 30–34%, showed that growth is reduced at 50 and 30% O2-saturation for both species during a 20 d period. There was some adaptation to hypoxia resulting in less reduction of growth during the second half of the experiment. The frequency of fish eating was reduced in plaice at 30% O2-saturation. Reduced mean total length of the plaice and dab population of the SE Kattegat is discussed in view of sublethal effects of oxygen deficiency.

Distribution and growth dynamics of ephemeral macroalgae in shallow bays on the Swedish west coast

Abstract Distribution and growth dynamics of ephemeral macroalgae were investigated in some shallow (0–1 m) bays on the Swedish west coast during the period 1992 to 1994. Variation in cover and biomass was assessed in nine bays, and in one of them the seasonal dynamics of these algae was followed intensively over three years. Frequent measurements were taken of algal biomass, degree of cover, in situ growth, variable fluorescence and C/N-ratios. Irradiance and water nutrient concentrations were measured concurrently with the growth measurements. Ephemeral macroalgae were dominated by Cladophora and Enteromorpha species and occurred in all sampled bays, except one, covering 10 to 100% of the bottom sediment. Generally, a rapid biomass increase was recorded from mid-May, which peaked after six weeks at 400–600 g dwt·m−2. Later in the season, strong variations in biomass, cover and species composition were observed, suggesting that these opportunistic algae form a highly dynamic community. Initial growth rates estimated from biomass samples were similar to those recorded from in situ cage experiments, and also agreed with growth rates calculated from a model. For all species studied growth rate was within the range 10 to 30 g dwt·m−2·d−1, irrespective of method used. Low algal C/N-ratios (mean = 12.7) in 1993 (cold and rainy summer) indicated that growth was not limited by nutrients, but rather by light. In 1994 (warm and sunny summer), mean C/N-ratios were 20, reflecting the opposite situation. The appearance of these opportunistic algae in shallow bays which historically had been without macroalgal communities has changed the characteristics of these areas by altering habitat complexity. This could have important consequences for trophic interactions involving many species, thereby altering community structure and function.

Abundance, biomass and production of mobile epibenthic fauna in Zostera marina (L.) meadows, western Sweden

Abstract Mobile epifauna was sampled quantitatively with a drop-trap in three Zostera marina (L.) meadows on the west coast of Sweden, 1980–82. Of 20 species recorded, 10 were crustaceans (mostly shrimps) and 10 fishes (gobies, pipe-fishes and sticklebacks). Crustaceans dominated in abundance and biomass throughout the season and in all areas. Total abundance and biomass varied between 20 and 160 ind. · m−2 and 1 and 12 g (ash-free dry weight, AFDW) · m−2. The shrimp Palaemon adspersus (Rathke) and the shore crab Carcinus maenas (L.) contributed 65 to 90% of the mean seasonal biomass in the three areas, and the dynamics of these two species are described in this paper. Production of the 6–8 dominant species during eight months (May–December) of investigation varied between 2.5 and 6.0 g AFDW · m−2. No relation was found between the biomass of fauna and standing stock of Zostera, but patches of Fucus vesiculosus in the Zostera were shown to play an important role for some species.

Continuous monitoring of dissolved oxygen in an estuary experiencing periodic hypoxia and the effect of hypoxia on macrobenthos and fish

ABSTRACT Hypoxic bottom water (defined as * ) is now a common occurrence in many estuarine and coastal systems. If prolonged and widespread, hypoxia could present a serious threat to the ecological balance of these systems. In areas where hypoxia is intermittent and not prolonged, its effect on macrobenthic behavior and energetics may facilitate transfer of energy to oxygen-tolerant bottom-feeding fish. This study examined changes in the behavior and populations of benthic fish and invertebrates that were associated with periodic hypoxia in the York River, Chesapeake Bay, USA. In the York River tidally driven neap–spring cycles of water column stratification–destratification, associated with the lunar cycle, are the primary factors regulating levels of bottom water dissolved oxygen during the summer. Automated continuous environmental data recording (every 20 min) was instrumental in understanding the response of the system to oxygen stress and allowed for timing of data collection at known times within a normoxic–hypoxic cycle. Data indicated that while macrobenthic community structure was insensitive to brief periods of hypoxia, behavior, growth, and production were affected. Demersal feeding fish changed their feeding habits quickly to take advantage of stressed macrobenthos that came to the sediment surface. The apparent color redox-potential discontinuity layer in the sediment slowly became shallower as summer progressed.

Regime Shifts and Ecosystem Service Generation in Swedish Coastal Soft Bottom Habitats: When Resilience is Undesirable

Ecosystems can undergo regime shifts where they suddenly change from one state into another. This can have important implications for formulation of management strategies, if system characteristics develop that are undesirable from a human perspective, and that have a high resistance to restoration efforts. This paper identifies some of the ecological and economic consequences of increased abundance of filamentous algae on shallow soft bottoms along the Swedish west coast. It is suggested that a successive increase in the sediment nutrient pool has undermined the resilience of these shallow systems. After the regime shift has occurred, self-generation properties evolve keeping the system locked in a high-density algae state. The structural and functional characteristics of the new system state differ significantly from the original one, resulting in less valuable ecosystem goods and services generated for society. In Sweden, loss of value results from the reduced capacity for mitigating further coastal eutrophication, reduced habitat quality for commercial fishery species, and the loss of aesthetic and recreational values.