Autumn Water Research Articles

Survival, growth and immune activity of scallop Chlamys farreri cultured at different depths in Haizhou Bay (Yellow Sea, China) during hot season

Survival, growth and immune response of the scallop, Chlamys farreri, cultured in lantern nets at five different depths (2, 5, 10, 15, and 20 m below the sea surface) were studied in Haizhou Bay during the hot season (summer and autumn) of 2007. Survival and growth rates were quantified bimonthly. Immune activities in hemolymph (superoxide dismutase (SOD) and acid phosphatase (ACP)) were measured to evaluate the health of scallops at the end of the study. Environmental parameters at the five depths were also monitored during the experiment. Mortalities mainly occurred during summer. Survival of scallops suspended at 15 m (78.0%) and 20 m (86.7%) was significantly higher than at 2 m (62.9%), 5 m (60.8%) or 10 m (66.8%) at the end of the study. Mean shell height grew significantly faster at 10 m (205.0 μm/d) and 20 m (236.9 μm/d) than at 2, 5 or 15 m in summer (July 9 to September 1); however, shell growth rate at 20 m was significantly lower than at the other four depths in autumn (September 2 to November 6). In contrast to summer, scallops at 5 m grew faster (262.9 μm/d) during autumn. The growth of soft tissue at different depths showed a similar trend to the shell. Growth rates of shell height and soft tissue were faster in autumn than in summer, with the exception of shell height at 20 m. SOD activity of scallops increased with depth, and ACP activity was significantly higher at 15 and 20 m than at other depths, which suggests that scallops were healthier near the bottom. Factors explaining the depth-related mortality and growth of scallops are also discussed. We conclude that the mass mortality of scallop, C. farreri, during summer can be prevented by moving the culture area to deeper water and yield can be maximized by suspending the scallops in deep water during summer and then transferring them to shallow water in autumn.

Habitat quality of a subarctic nursery ground for 0-group plaice ( Pleuronectes platessa L.)

Habitat quality of a subarctic nursery ground in northern Norway for 0-group plaice Pleuronectes platessa was investigated by following settlement, mortality and growth during 2005 and 2006. Newly settled individuals were first observed in the end of May to early June and settlement lasted until mid-July. Densities peaked in early July and were comparable to those reported in temperate nursery grounds. Mortality estimates after settlement differed between 0.062 d − 1 in 2005 and 0.025 d − 1 in 2006. Potential predators appeared to be rather similar as those reported in other areas: the brown shrimp Crangon crangon, the shore crab Carcinus maenas and demersal fish species (gadoids). Population mean growth indicated linear growth until August leveling-off afterwards. 0-group plaice reached a lower mean size (5–6 cm) at the end of the growing season than in temperate areas probably due to later settlement timing in combination with lower summer–autumn water temperatures. The comparison of observed growth rates with predictions of maximum growth models indicated a similar pattern as observed in temperate nursery grounds: Growth appeared to be maximal except for the period after summer. Whether or not this was related to changes in food quality throughout the season, to interspecies competition or to emigration remains to be elucidated.

Sperm motility in European hake, Merluccius merluccius, and characterization of its spermatozoa concentration and volume, spermatocrit, osmolality and pH

Due to stock declines there is a growing interest in farming of European hake Merluccius merluccius. Thus, knowledge of its sperm biology is of importance not only for purposes of broodstock management, but also for the development of sperm preservation techniques. Hake sperm were collected from mature males caught during the summer–early autumn waters off western Norway and during the winter–early spring in the Bay of Biscay (France). Sperm quality characteristics were assessed after storage at 4 °C for 25 ± 14 h. Average (± SD) values for Norwegian and French samples respectively were (i) sperm volume: 3.9 ± 4.0 and 2.6 ± 4.0 ml; (ii) spermatozoa concentration: 6.6 ± 3.2 and 13.9 ± 5.1 × 10 9 spermatozoa/ml; (iii) spermatocrit: 80.2 ± 3.3 and 81.8 ± 10.7%; and (iv) total number of spermatozoa: 23.5 ± 30.0 and 35.1 ± 36.2 × 10 9. Average osmolality and pH (± SD) of French samples were 349 ± 28 mOsmol/kg and 7.6 ± 0.1, respectively. Activation by transfer into full sea water (100 SW) or 10% ovarian fluid in sea water (10 OF) occurred synchronously for virtually all spermatozoa and the percent sperm motile decreased with post activation time. When transferred into 50% sea water diluted with distilled water (50 SW) only a few spermatozoa were activated initially but subsequently reached a maximum percentage of motility followed by a decline. Hake sperm motility declined rapidly to 50% of motility 70 s after activation with 100 SW. Sperm were motile for longer when activated with 50 SW (1570 ± 295 s) or 10 OF (718 ± 71 s) compared to 100 SW (317 ± 121 s). Undiluted hake sperm stored at 4 °C up to 10 days retained 10% motility when activated with 100 SW. When cryopreserved, the motility recovery index of the cells at thawing ranged from 0 to 76.4%. These results describe for the first time the sperm traits of European hake following successful cryopreservation, and also show the importance of activation medium on sperm motility.

Larval fish assemblages and circulation in the Eastern Tropical Pacific in Autumn and Winter

In this work, we linked larval fish assemblages with water masses and circulation in the Eastern Pacific off Mexico, during autumn 2005 and winter 2007. Four assemblages were defined. (i) The Transitional assemblage, with the lowest mean larval abundance and dominated by tropical mesopelagic Vinciguerria lucetia and Diogenicthys laternatus. It was associated with modified California Current Water in winter and with modified Surface Water in autumn. (ii) The Coastal-oceanic assemblage was found off Cabo Corrientes, with high larval abundance, and dominated by Bregmaceros bathymaster; part of this assemblage was trapped by coastal cyclonic eddies. (iii) The Tropical A assemblage was associated with Surface Water. It had the highest abundance and richness, and the largest number of dominant species (e.g. D. laternatus, Auxis spp.); it covered a wider area in winter than in autumn. (iv) The Tropical B assemblage, distinguished by the highest abundance of V. lucetia, was present only in autumn; it was associated with overall anticyclonic circulation of warm Surface Water. The agreement between larval fish assemblage distributions, water masses and mesoscale dynamics indicates that the formation and permanence of assemblages depends on the interaction of spawning strategies of different species with large-scale and mesoscale processes.

Bitlis İli İçme Sularının Bazı Mikrobiyolojik ve Fizikokimyasal Özellikleri

In the present study, microbiological and physicochemical qualities of drinking water obtained from the city of Bitlis and its provinces (Adilcevaz, Ahlât, Guroymak, Hizan and Tatvan) were studied. A total of 164 water samples, collected from tank and tap waters in spring and autumn, were used as material. Aerobic plate counts were carried out by pour plate technique. While membrane filtration method was used for the counts of enterococcus, coliform, E. coli and sulphite reducing anaerobic bacteria and standard methods were recruited for physicochemical analysis. The results of physicochemical analysis of water samples were found to be in conformity with the standard regulations. Microbiological results of water, however, did not comply with required standards since the findings showed 30% enterococci, 12% coliform, 24% sulphite reducing anaerobic and 8% E. coli. The mean values of pH, Na and Mg, and presence of bacteria except E. coli were higher in autumn than spring. Effect of residential areas on aerobic plate count, coliform and E. coli were found to be insignificant (p>0.05), the effects on all other variables were found to be significant (p<0.05).

Physiology, growth, and development of larval krill Euphausia superba in autumn and winter in the Lazarev Sea, Antarctica

The physiological condition of larval Antarctic krill was investigated during austral autumn 2004 and winter 2006 in the Lazarev Sea. The condition of larvae was quantified in both seasons by determining their body length (BL), dry weight (DW), elemental and biochemical composition, stomach content analysis, and rates of metabolism and growth. Overall the larvae in autumn were in better condition under the ice than in open water, and for those under the ice, condition decreased from autumn to winter. Thus, growth rates of furcilia larvae in open water in autumn were similar to winter values under the ice (mean, 0.008 mm d−1), whereas autumn underice values were higher (0.015 mm d−1). Equivalent larval stages in winter had up to 30% shorter BL and 70% lower DW than in autumn. Mean respiration rates of winter larvae were 43% lower than of autumn larvae. However, their ammonium excretion rates doubled in winter from 0.03 to 0.06 µg NH4 DW−1 h−1, resulting in mean O:N ratios of 46 in autumn and 15 in winter. Thus, differing metabolic substrates were used between autumn and winter, which supports a degree of flexibility for overwintering of larval krill. The larvae were eating small copepods (Oithona spp.) and protozoans, as well as autotrophic food under the ice. The interplay between under‐ice topography, apparent current speed under sea ice, and the swimming ability of larval krill is probably critical to whether larval krill can maintain position and exploit suitable feeding areas under the ice.

Assessment of the Amur River Ecosystem Pollution with Benzene and Its Derivatives Caused by an Accident at the Chemical Plant in Jilin City, China

Assessment of concentrations of benzene and its derivatives in the Amur water and fish caught in summer and autumn 2006 was carried out after Jilin chemical plant accident (winter 2005) in the People’s Republic of China. Fish sampling of Leiocassis ussuriensis conducted in the Amur River near Khabarovsk identified benzene, toluene, ethylbenzene, isopropylbenzene, xylene reaching to 16, 98.7, 55, 54, and 206 μg kg−1, respectively. For Hypophthalmichtys molitrix Valenciennes sampled in the lower reaches of the Amur benzene, toluene, ethylbenzene, xylene were found in amounts to 9, 15.4, 16 μg kg−1, and, benzene and xylene reaching 6 and 16 μg kg−1 were found in Huso dauricus and benzene and xylene of 2–6 and 13–16 μg kg−1, respectively, in Acipenser schrenckii. The above toxicants were not detected in the Amur water near the river mouth in autumn 2006.

Investigation of first year biotic and abiotic influences on the recruitment of pikeEsox luciusover 48 years in Windermere, U.K.

Estimated pike Esox lucius recruitment varied by a factor of 16 for females from 1944 to 1991 and by a factor of 27 for males from 1943 to 1990 in Windermere, a temperate, mesotrophic U.K. lake. No significant stock-recruitment relationships were found, but analysis with general additive models (GAMs) revealed that early autumnal water temperature, strength and direction of the North Atlantic Oscillation displacement (corresponding to different climatic conditions in winter) and zooplankton abundance but above all, late summer water temperature were important explanatory variables over the entire time series. Female recruitment was also influenced by young-of-the-year winter temperature. There was no evidence that perch Perca fluviatilis year-class strength, lake level or the summer position of the Gulf Stream influenced recruitment. The fitted models explained up to c. 65% of the overall observed variation between years.

Transequatorial Migrations by Basking Sharks in the Western Atlantic Ocean

The world's second largest fish, the basking shark (Cetorhinus maximus), is broadly distributed in boreal to warm temperate latitudes of the Atlantic and Pacific oceans from shallow coastal waters to the open ocean. Previous satellite archival tagging in the North Atlantic has shown that basking sharks move seasonally, are often associated with productive frontal zones, and may make occasional dives to mesopelagic depths. However, basking sharks are thought to be restricted to temperate latitudes, and the extent to which they exploit deeper-water habitat remains enigmatic. Via satellite archival tags and a novel geolocation technique, we demonstrate here that basking sharks are seasonal migrants to mesopelagic tropical waters. Tagged sharks moved from temperate feeding areas off the coast of southern New England to the Bahamas, the Caribbean Sea, and onward to the coast of South America and into the Southern Hemisphere. When in these areas, basking sharks descended to mesopelagic depths and in some cases remained there for weeks to months at a time. Our results demonstrate that tropical waters are not a barrier to migratory connectivity for basking shark populations and highlight the need for global conservation efforts throughout the species range.

Spatial and temporal variations in nutrient and chlorophyll- a concentrations in the northern East China Sea surrounding Cheju Island

Nutrients, chlorophyll- a (Chl -a), and environmental conditions were extensively investigated in the northern East China Sea (ECS) near Cheju Island during five research cruises from 2003 to 2007. In the eastern part of the study area, surface waters were characterized only by the Tsushima Current Water (TCW) during all five cruises. However, the western surface waters changed with season and were characterized by the Yellow Sea Cold Water (YSCW) in spring, the Changjiang Diluted Water (CDW) in summer, and the Yellow Sea Mixed Water (YSMW) in autumn. In spring and autumn, relatively high concentrations of nitrate and phosphate were observed in the surface waters in the western part of the study area, where vertical mixing brought large supplies of nutrients from deep waters. Changes in wind direction occasionally varied the inflow of the Changjiang plume in summer, clearly causing the annual variation in surface nitrate and phosphate concentrations in summer. In summer, the surface distribution of nitrate and phosphate did not coincide with that of silicate in the study area, which probably resulted from the significant drop in the Si:N ratio in the Changjiang plume since construction of the Three Gorges Dam (TGD). Despite large temporal and spatial variations in surface Chl- a concentrations, depth-integrated Chl- a concentrations exhibited little variation temporally and spatially. In the study area, surface Chl- a concentration did not well reflect the standing stocks of phytoplankton. The vertical distribution of Chl- a showed large temporal and spatial variations, and the main factor controlling the vertical distribution of Chl- a in summer was the availability of nitrate. The thermohaline front may play an important role for accumulation of phytoplankton biomass in spring and autumn.

Emigration patterns among trout, Salmo trutta (L.), kelts and smolts through spillways in a hydroelectric dam

Abstract The emigration patterns among radio‐tagged trout, Salmo trutta L., kelts (n = 41, total length: 60–90 cm) and smolts [n = 27, total body length (BL): 22–30 cm] in the regulated River Gudbrandsdalslågen, south‐east Norway, were studied by investigating the influence of sex (kelts) and BL (kelts and smolts) on the timing of emigration. In total, 49% of the kelts emigrated towards the hydroelectric dam shortly after spawning, whilst 51% over‐wintered. Female kelts were five times more likely to initiate autumn emigration, and eight times more likely to descend the spillways during the first release of surface water than males. Large individuals of both sexes descended earlier than smaller individuals. Larger smolts were more likely to descend during the first release of surface water than smaller smolts. To safeguard the emigration of iteroparous trout kelts and smolts, the spillways should release surface water both in autumn and spring to avoid selective forces during emigration through spillways.

Microbial community of freshwater sponges in Lake Baikal

The microbial community of Baikal sponges has been studied in five species belonging to the genera Swartschewskia, Baicalospongia, and Lubomirskia of the endemic family Lubomirskiidae. The results show that the total numbers of bacteria and bacterioplankton production have an effect on the growth of L. baicalensis body. Bacteria of the genera Pseudomonas, Bacillus, Micrococcus, Sarcina, Flavobacterium, Arthrobacter, and Acinetobacter living in the sponges are representatives of the Baikal bacterioplankton. Actinomycetes of the genera Streptomyces and Micromonospora are a permanent component of the cultivable sponge microbial community. The numbers and enzyme activities of heterotrophic, oligotrophic, and psychrophilic bacteria isolated from different sponge species and the surrounding water in autumn and in winter have been estimated.

The distribution and abundance of the Neogobius fishes in their native range (Bulgaria) with notes on the non-native range in the Danube River

The recent distribution and abundances of the genus Neogobius were studied (using electrofishing and beach seining) within (Bulgaria) and outside (Austria, Slovakia, Croatia) its native range along the longitudinal profile of the Danube River in 2004, 2005 and 2006. The sampling was conducted under normal autumn water levels. CPUE values (number of fish per 100 m shoreline) were calculated and compared between sampling sites. The presence of four Neogobius species - N. kessleri, N. fluviatilis, N. melanostomus, and N. gymnotrachelus -was confirmed in Bulgaria and Croatia. Though the latter two species did occur in Croatia, they occurred at very low densities (comparing to the densities in the upper and lower Danube), indicating newly established populations. Our results support the hypothesis of disjunct spreading of these species. The presence of N. fluviatilis in Austria was not confirmed. In general, the Neogobius spp. showed the lowest density within their native range (6 individuals per 100 m shoreline) while their densities were considerably larger outside their native range (18-28 individuals per 100 m shoreline). In addition. N. kessleri and N. melanostomus were significantly larger at sites of non-native distribution compared to native sites.

Multiannual and Seasonal Variability of Water Masses and Fronts Over the Uruguayan Shelf

Abstract Seasonal (autumn and spring) and multiannual (1991, 1995, 1996, 1997, 1998, and 2001) variability in the composition of water masses on the Uruguayan shelf were analyzed. The main difference between seasons was the presence of Tropical Water (TW) and a relative dominance of Subtropical Water (STW) over Subantartic Water (SAW) in autumn, whereas spring was characterized by a dominance of SAW and the absence of TW. Changes in the composition of water masses, surface temperature distributions, and front positions and strengths are discussed in regards to biological implications. Our results reveal seasonal changes in the frontal zone position and suggest an enhaced migration during El Nino Southern Oscillation (ENSO) warm and cold episodes, reaching higher latitudes during the former (e.g., autumn of 1998). During cold episodes, the position of the frontal zone reached lower latitudes, with prevailing cold waters over the shelf. Higher frontal strength was observed during ENSO episodes, mainly becau...

Inferred Phosphorus Cycling in Shallow and Deep Basins of Eutrophied Lake Hormajärvi, Southern Finland

Lake Hormajärvi is a typical small lake in southern Finland suffering from human-induced eutrophication. The lake has a complex basin morphology; the western basin is deeper and does not suffer the consequences of eutrophication of the shallower eastern basin, where poor water quality prevents swimming, fishing and other recreational use of the lake. Two short sediment cores from both basins were taken and studied to understand the eutrophication history of the lake, phosphorus (P) cycling within the respective basins and to evaluate needs for lake management and restoration. Sediments were analyzed for diatoms, P fractions, and physical properties. Spherical carbonaceous particles (SCP) were used for dating. Results indicate that diatom inferred autumnal lake surface water total P (DITP) does not follow the trend in accumulated P in the western basin, whereas there is a clear correlation in the eastern basin. The western basin acts as a permanent sink for P, but in the unstratified eastern basin, P released from sediment circulated throughout the water column. Results demonstrate that lake management or restoration decisions should not be made based on water quality and algal bloom observations alone. Detailed paleolimnological studies provide essential information to understand processes in complicated basins like Lake Hormajärvi.

Latitudinal variation in the recruitment dynamics of small pelagic fishes in the western North Pacific

The subarctic Oyashio Current flows south-westward and the subtropical Kuroshio Current flows north-eastward in the western North Pacific, converging in the waters off northern Japan to form the Kuroshio-Oyashio transition region. Some small pelagic fishes inhabit the subarctic or subtropical waters, and others seasonally migrate north and south across the major ocean fronts. Environmental conditions in the subarctic and transition waters are variable, whereas in the subtropical Kuroshio waters conditions are relatively stable. Latitudinally different environmental conditions may affect vital parameters and recruitment variability of small pelagic fishes inhabiting the various waters. Pacific saury Cololabis saira migrate seasonally from the Kuroshio to Oyashio waters and spawn in the transition waters in autumn and spring and in the Kuroshio waters in winter. During 1990–1999, the coefficients of variation (CVs) of daily growth rates (G) and instantaneous mortality coefficients (M) were large for larvae and juveniles spawned in the northern transition waters, but relatively small for those from the southern Kuroshio waters. The Pacific stock of chub mackerel Scomber japonicus spawns in the Kuroshio waters in spring and early summer and migrates to the subarctic Oyashio waters in summer for feeding, whereas the Tsushima Warm Current stock spawns in the East China Sea in spring and fish remain in the subtropical warm waters throughout their lifetime. The Pacific stock had CVs > 100% for the fish aged 0–5 during 1970–2002. In contrast, the Tsushima Warm Current stock had CVs of 34–40% during 1973–2002. Pacific herring Clupea pallasii, which inhabits subarctic waters, had CVs of 118–178% for the fish aged 3–8 y during 1910–1954. Japanese sardine Sardinops melanostictus, which spawn in the subtropical Kuroshio waters and migrate to the subarctic Oyashio waters in summer for feeding, had CVs > 120% for the fish aged 0–4 during 1976–2003. Contrasting with these subarctic Oyashio associated species, subtropical round herring Etrumeus teres had a CV of 26% over the total catch during 1976–2003. For these small pelagic fishes, latitudinal variation in early life parameters and recruitment was evident at the levels of cohorts ( C. saira), stocks ( S. japonicus), and species (Clupeidae). Large variations were observed in the subarctic Oyashio and the Oyashio-affected transition waters, whereas relatively stable results were recorded for the subtropical Kuroshio and its branch, the Tsushima Warm Current. Because the cohorts of Pacific saury and stocks of chub mackerel are not genetically distinct, it is likely that the large variability in the northern waters was environmentally induced in these species. The differences in CVs among clupeids are probably the result of differences in specific reproductive strategies and the environmental characteristics of the waters inhabited by the different species.

A simple empirical model for remote sensing reflectance of Lake Taihu waters in autumn

Remote sensing reflectance,backscattering and other corresponding parameters were measured in situ in Lake Taihu mainly following with NASA protocols.And suspended particulate matter (SPM) concentrations were measured in the lab mainly following with lake investigation criterion of China.Firstly,the whole lake was divided into three areas based on the literature's results,where the remote sensing reflectance was influenced,not influ- enced,and possibly influenced by the lake bottom,respectively.They were named by the influenced area,the non-influenced area,and the possibly influenced area,respectively.Then,we divided the whole waters into two types and four sub-types based on the Secchi disk transparency measured in situ,viz.the turbid water,including the high-turbid water and the mid-turbid water,and the clear water,including the low-clear water and the high- clear water.And their properties were described by the corresponding data measured in situ or in the lab.In the whole lake,the empirical model was developed between the backscattering coefficient and SPM concentration.Only in the non-influenced area was developed the empirical model between the pairs of the remote sensing reflectance and backscattering coefficient.The remote sensing reflectance can be estimated approximately by SPM concentra- tion and the estimation models were developed based on the mentioned above.Finally,the model precisions were analyzed in detail.Maybe it is a very good approach to preliminarily know about apparent optical properties of wa- ters in Lake Taihu when there are not optical data measured in situ but only SPM concentrations measured in the lab.

A methodological approach to characterize the resilience of aquatic ecosystems with application to Lake Annecy, France

We propose a methodological approach to characterize the resilience of aquatic ecosystems with respect to the evolution of environmental parameters as well as their aptitude to adapt to forcings. This method that is applied to Lake Annecy, France, proceeds in three stages. First, according to the depth, variations of physicochemical parameters versus time are separated into three components related to (1) energy transfer through the surface of the lake, (2) the flow of rivers and springs that feed the lake, and (3) long‐term evolution of the benthic zone as a consequence of mineral and organic matter loads. Second, dynamics of the lake are deduced by analyzing the physicochemical parameter components related to the three boundary conditions. Third, a stochastic process associated with the transfer models aims to characterize the resilience of the lakes according to forcings. For Lake Annecy, whose dynamics are representative of oligotrophic stratified lakes controlled by decarbonation processes where turnover and mixing occurring once a year in winter, the major consequence is the impoverishment of dissolved oxygen in deep water in autumn due to a temperature increase of the surface water in summer. The simulation raises relevant questions about whether a connection exists between physicochemical parameters and global warming, which should not induce harmful consequences on water quality and biodiversity in deep water. This methodological approach is general since it does not use any physical conceptual model to predict the hydrosystem behavior but uses directly observed data.

Potential capacity of coprecipitation of dissolved organic carbon (DOC) with iron(III) precipitates

Hypolimnetic anoxic water of Lake Onogawa was subjected to aeration experiments. When the samples were agitated by magnetic stirrers for 24 h, dissolved oxygen increased from 0 to more than 7.6 mg l−1, dissolved iron decreased from 98% to about 5% of the initial total iron, and from 32% to 48% of the dissolved organic carbon (DOC) disappeared. On the other hand, when the anoxic waters were left unstirred, dissolved oxygen increased from 0 to 2.2 mg l−1, dissolved iron decreased from 98% to 31%, and 20% of the DOC disappeared within 48 h. Further 24-h incubation had little effect on the DOC loss, although dissolved oxygen increased to 3.9 mg l−1 and dissolved iron decreased to 5%. These rates of DOC disappearance are too large to be explained by bacterial decomposition. It is quite conceivable that a part of the DOC is coprecipitated with iron(III) precipitates. When Fe(II) in the anoxic hypolimnion is oxidized by autumnal water mixing, probably anoxic water is mixed with aerobic water. The anoxic water must receive oxygen from the aerobic water during this mixing and be simultaneously diluted with the aerobic water. Because the present experimental conditions, especially the stirred one, significantly differ from in situ conditions, the present results are thought to be a potential capacity of DOC coprecipitation.

Overgrazing of edible algae as a mechanism behind red tides and harmful algal blooms

It has recently been suggested that there is a positive synergistic relationship between herbivorous zooplankton and their algal food, with herbivores stimulating the production of their preferred species while negatively affecting competing algal species. A prerequisite for this positive interaction hypothesis is that herbivores and phytoplankton coexist in balance in which recycled nutrients from herbivores are immediately utilised by phytoplankton, which, in turn, are grazed by the herbivores. Empirical evidence suggests that this balance is more stable during increasing primary productivity than during its decline. During longer periods of highly stratified water in summer and early autumn, primary productivity may decrease because the export of nutrients from the euphotic zone outweighs import. If there is a lag between the decline in primary production and the decline in herbivore biomass, the herbivores may overgraze their preferred algal species, which, in turn, cannot fully utilise the recycled nutrients. Hence, nutrients may become available for non-grazed phytoplankton species that cause red tides and harmful algal blooms (HABs). In other words, it is hypothesised that HABs and red tides are caused by the collapse of the balanced positive (synergistic) relationship between herbivores and their preferred algal food.