In the last ~15 yr, frequent recruitment failures in the main bivalve species in the western Wadden Sea led to insufficient food supply for some specialised bird species, sharpening conflicts be- tween nature conservationists and fishermen. To study possible causes of the recent recruitment failure in bivalves, we compare long-term data sets (1973 to 2002) of annual abundance of spat of 3 of the most important species of bivalves (cockle Cerastoderma edule, gaper clam Mya arenaria, and Baltic tellin Macoma balthica) on Balgzand, a tidal-flat area in the westernmost part of the Wadden Sea. In the 3 spe- cies, recruitment success declined significantly over the period of observation, particularly at offshore sampling sites which were characterized by low intertidal levels and sandy sediments. In these areas, we found high biomass values of a predator of bivalve postlarvae, the shrimp Crangon crangon. In each of the 3 bivalve species, annual recruitment (estimated as numerical density of spat in August) in these areas was negatively related to shrimp biomass at the time of settlement of postlarvae (May/June). Shrimp biomass has increased over the last ~30 yr. High near-shore flats showed invariably low shrimp biomass values and appear to serve as a refuge for postlarval bivalves in years of high predation pres- sure. Only in this coastal part of Balgzand was no decline in bivalve recruitment found; in fact, cockle re- cruitment even increased. Alternative explanations for the observed changes in recruitment of bivalves are discussed, including changes in sediment composition and bottom-disturbing fishing for cockles, mussels and lugworms. It is concluded that the recruitment trends on Balgzand (and other parts of the Wadden Sea) are governed primarily by natural processes, in particular increases in predation pressure on early benthic stages, which in turn appears to be largely governed by the warming climate.

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Decline of recruitment success in cockles and other bivalves in the Wadden Sea: possible role of climate change, predation on postlarvae and fisheries

We show that death by dissolution is an important size-dependent mortality factor for juvenile bivalves. Utilizing a new experimental design, we were able to replicate saturation states in sediments after values frequently encountered by Mercenaria mercenaria in coastal deposits (Varagonite 5 0.4 and 0.6). When 0.2-mm M. mercenaria were reared in sediments at Varagonite 5 0.4 and 0.6, significant daily losses of living individuals occurred (14.0% and 14.4% d21, respectively), relative to supersaturated-control sediments (3.9% d21). For 0.4mm M. mercenaria, significant mortality occurred under the most undersaturated conditions (Varagonite 5 0.4, mortality 5 9.6% d21), although mortality at Varagonite 5 0.6 was not significant (mortality 5 2.7% d21; controlsaturated mortality 5 0.2% d21). For the largest size-class investigated, 0.6 mm, we show significant mortality for clams under the most undersaturated sediments (Varagonite 5 0.4, 2.8% d21). To test if buffered sediments would increase survivorship of juvenile bivalves during periods of recruitment, we manually manipulated sediment saturation state by adding crushed Mya arenaria shell to a mud flat in West Bath, Maine, U.S.A. Although we increased the average sediment saturation state within retrieved cores from V 5 0.25 6 0.01 to only 0.53 6 0.06, numbers of live M. arenaria in buffered sediment increased almost three-fold in 2 weeks. Buffering muds against the metabolic acids that cause lowered saturation states may represent a potentially important management strategy to decrease dissolution mortality. Shallow-water deposits can be significantly undersaturated with respect to both calcite and aragonite and dissolution may occur in surface sediments where juvenile

/pdf/death-by-dissolution-sediment-saturation-state-as-a-16lp29qsr4.pdf

Death by dissolution: Sediment saturation state as a mortality factor for juvenile bivalves

We evaluate experimentally the effect of carbonate saturation state at the sediment-water interface (SWI) on survivorship of various size classes of the juvenile bivalve Mercenaria mercenaria. Populations of 0.2-mm, 0.3-mm, 1-mm, and 2-mm M. mercenaria were introduced to sediments realistically undersaturated (experimental, saturation state with respect to aragonite = Ώaragonite = IMP/K′sp = ~0.3) and saturated (control, Ώaragonite = ~1.5) with respect to aragonite in order to evaluate the impact of saturation state and dissolution on survivorship. Linear regression analysis was used to examine mortality within each treatment over time and show significant mortality for each size class in experimental-undersaturated treatments only (P < 0.05). Mortality rates in experimental undersaturated sediments were -11.8, -4.8, -1.9, and -1.1% d−1 for the 0.2-, 0.3-, 1.0-, and 2.0-mm bivalves, respectively. Analysis of covariance (ANCOVA) was used to examine differences in mortality between treatments over time and show significantly different mortality rates for the 0.2-, 0.3-, and 1-mm individuals (P < 0.05). Dissolution may represent a previously unrecognized yet significant source of mortality for "just-set" juvenile bivalves, particularly the very small individuals that have been largely ignored in recruitment studies to date. Dissolution-induced mortality may help explain the exponential losses of juvenile bivalves following their transition from the pelagic larval phase to the benthic juvenile phase.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2004.49.3.0727

Dissolution mortality of juvenile bivalves in coastal marine deposits

Recent studies in terrestrial, plant-dominated systems have shown that reductions in diversity can affect essential ecosystem processes, especially productivity. However, the exact form of the relationship between diversity and ecosystem functions remains unknown, as does the relevance of these studies to other systems. We studied the relationships between macroinvertebrate species richness and ecosystem functions in a soft-bottom, intertidal system. We also considered, as a separate variable, the effects of macroinvertebrate biomass on ecosystem functions. A field experiment was conducted at Blackness, a mudflat in the Firth of Forth, Scotland, United Kingdom, using cages with different mesh sizes (195, 300, and 3000 μm) to establish low, medium, and high species richness treatments through differential colonization of defaunated sediments. Low, medium, and high biomass treatments were established by enclosing differing amounts of ambient sediment in defaunated plots. Other treatments controlled for the effects of defaunation and caging. The experiment ran for six weeks in the summer of 1999. All treatments contained species within the same five main functional groups of macroinvertebrate, but species' identity varied both within and between treatments (thus species richness was considered a random, rather than fixed, variable). A total of 27 macroinvertebrate species were sampled across all treatments; 37% of these occurred in the low, 52% in the medium, and 74% in the high diversity treatments.

At the end of the experiment, the following physical variables were measured as indicators of ecosystem functions such as sediment stabilization and nutrient fluxes: sediment shear strength (a measure of sediment cohesiveness), water content, silt/clay content, organic content, redox potential (a measure of anoxia), nitrate, nitrite, phosphate and ammonium fluxes, and community respiration. Changes in biomass and species richness were found to have significant effects on oxygen consumption; these relationships were driven in particular by the presence of the largest species in our study, Nephtys hombergii. All other variables were not significantly affected by the treatments. These results support the null hypotheses of no relationship between ecosystem functions and diversity and biomass. However, our experiment was necessarily limited in both spatial and temporal scale; the implications of this when scaling up to larger scale generalizations are discussed. Our results suggest that diversity/biomass/ecosystem function relationships in the soft sediment benthos are likely to be very complex and may depend more on functional groups than species richness.

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Diversity, biomass and ecosystem processes in the marine benthos.

Predation, in the broad sense of an organism killing another organism for nutritional purposes, is probably as old as life itself and has originated many times during the history of life. Although little of the beginnings is caught in the fossil record, observations in the rock record and theoretical considerations suggest that predation played a crucial role in some of the major transitions in evolution. The origin of eukaryotic cells, poorly constrained to about 2.7 Ga by geochemical evidence, was most likely the ultimate result of predation among prokaryotes. Multicellularity (or syncytiality), as a means of acquiring larger size, is visible in the fossil record soon after 2 Ga and is likely to have been mainly a response to selective pressure from predation among protists. The appearance of mobile predators on bacteria and protists may date back as far as 2 Ga or it may be not much older than the Cambrian explosion, or about 600 Ma. The combined indications from the decline of stromatolites and the diversification of acritarchs, however, suggest that such predation may have begun around 1 Ga. The Cambrian explosion, culminating around 550 Ma, represents the transition from simple, mostly microbial, ecosystems to ones with complex food webs and second- and higher-order consumers. Macrophagous predators were involved from the beginning, but it is not clear whether they originated in the plankton or in the benthos. Although predation was a decisive selective force in the Cambrian explosion, it was a shaper rather than a trigger of this evolutionary event.

Michael G Richards

Papers

Predation: a causal mechanism for variability in intertidal bivalve populations

Can postlarval bivalves select sediment type during settlement? A field test with Macoma balthica (L.) and Cerastoderma edule (L.)

The effects of the adult density of Macoma balthica on the recruitment of juvenile bivalves: a field experiment