Pelagic and benthic net production of dissolved inorganic carbon in an unproductive subarctic lake

1. Both the pelagic and benthic net dissolved inorganic carbon (DIC) productions were measured in situ on four occasions from June to September 2004, in the unproductive Lake Diktar-Erik in subarctic Sweden. The stable isotopic signal ( δ ¹³C) of respired organic material was estimated from hypolimnion water data and data from a laboratory incubation using epilimnion water.
2. Both pelagic and benthic habitats were net heterotrophic during the study period, with a total net DIC production of 416 mg C m⁻² day⁻¹, of which the pelagic habitat contributed approximately 85%. The net DIC production decreased with depth both in the pelagic water and in the sediments, and most of the net DIC production occurred in the upper water column.
3. Temporal variations in both pelagic and benthic DIC production were small, although we observed a significant decrease in pelagic net DIC production after the autumn turnover. Water temperature was the single most important factor explaining temporal and vertical variations in pelagic DIC production. No single factor explained more than 10% of the benthic net DIC production, which probably was regulated by several interacting factors.
4. Pelagic DIC production, and thus most of the whole-lake net production of DIC, was mainly due to the respiration of allochthonous organic carbon. Stable isotope data inferred that nearly 100% of accumulated DIC in the hypolimnion water had an allochthonous carbon source. Similarly, in the laboratory incubation using epilimnion water, c. 85% of accumulated DIC was indicated to have an allochthonous organic carbon source.     

Diel interactions between prey behaviour and feeding in an invasive fish, the round goby, in a North American river

1. We studied the diet of the invasive round goby (Neogobius melanostomus) on a diel basis in the Flint River, a warmwater stream in Michigan, U.S.A. Diet and available prey samples were collected seven times over a 24 h period in four consecutive months. The section of river studied lacked zebra mussels (Dreissena polymorpha), the primary prey of adult round gobies elsewhere in the Great Lakes region. 2. Diet changed on a diel basis with hydropsychid caddisfly and chironomid larvae predominating during the day, chironomid pupae dominating in the evening and heptageniid mayflies dominating at night. Simultaneous study of macroinvertebrate drift suggested that caddisfly and chironomid larvae were most likely picked from submerged rocks, chironomid pupae were most likely taken during their emergent ascent and mayflies were either captured from the drift or picked from rocks. 3. The Flint River lacks a diverse darter (Family: Percidae) and sculpin (Family: Cottidae) fauna and it appears that the round goby has occupied a generalised darter/sculpin niche. Our results indicate that round gobies have the potential to invade successfully riverine systems, particularly those lacking a diverse benthic fish assemblage.     

Factors affecting nest defence in female Canada Geese Branta canadensis

The effect of various characteristics of Canada Goose Branta canadensis females and clutch size on their nest defence intensity during incubation was studied in the field during five nesting seasons. Nests were approached by a human, and the leaving distance was measured. Risk-taking in post-encounter behaviour was categorized at three levels. Nesting success was significantly associated with nest tenacity, a variable derived from leaving distance, but not with post-encounter behaviour. Nest tenacity, as well as post-encounter behaviour, differed significantly between individuals. Intensity of post-encounter behaviour, but not nest tenacity, increased with increasing age of the female. Nest tenacity, standardized over the incubation period, increased with increasing clutch size, but post-encounter behaviour intensity did not. None of the nest defence variables was correlated with the structural size of the female. Nest tenacity, but not post-encounter behaviour intensity, increased during incubation. Nest tenacity did not differ between early and late breeders, but post-encounter behaviour was more intense in late breeders. There was no evidence of positive reinforcement of nest defence through repeated visits. Nest defence did not differ between lakes, lake types or nest site types, nor was it affected by the distance to the nearest neighbouring nest. The results supported several of the predictions extracted from parental investment theory.     

Food selection, food-seeking patterns and hunting success of captive Goosanders Mergus merganser and Red-breasted Mergansers M. serrator in relation to the behaviour of their prey

Food selection and food-seeking behaviour of hand-raised Goosanders Mergus merganser and Red-breasted Mergansers M. serrator were tested in experimental situations. When different combinations of two species of fish were presented simultaneously to hungry birds in a small trough where the prey was easily and equally available, neither duck species showed any preferences in prey selection. However, when these same fish species were presented to satiated birds, they showed definite preferences; the most preferred species were baltic salmon and brown trout followed in decreasing order by minnow, whitefish, sculpin, burbot and river lamprey. This was also the case when combinations of five to seven species were presented. However, under semi-natural conditions in a stream tank, the number of each species offish caught was related to their escape behaviour, not to the previously determined predator preferences. When salmon and minnows of different size were presented to hungry birds, they selected the larger fish regardless of species. In contrast, when satiated, they preferred small fish over large fish. Under the experimental conditions, the use of certain elements of foraging behaviour clearly differed between the two bird species: Mergus serrator spent more time hunting underwater than did M. merganser. Results are discussed in relation to the predator-prey situation existing under natural conditions in northern Swedish rivers.     

Morphological reinforcement,ancient introgressive hybridization and species delimitation in African stem‐borer species of the genus Sesamia Guenée (Lepidoptera: Noctuidae)

Species delimitation, an issue central to systematics and biodiversity studies, is addressed in the epunctifera group of the stem borer genus Sesamia Guenée (Lepidoptera: Noctuidae). This group is composed of four sub‐Saharan species: Sesamia poephaga Tams & Bowden; Sesamia epunctifera Hampson; Sesamia penniseti Tams & Bowden; and Sesamia poebora Tams & Bowden, the taxonomic status of which was unclear. The first species was considered a possible synonym of the second, and the third species was considered a possible synonym of the fourth. An analysis combining morphological, ecological and molecular data enables us to conclude that S. epunctifera and S. poephaga are different species, and that S. poebora is a synonym of S. penniseti. Two new species were discovered: Sesamia firmata sp.n. and Sesamia veronica sp.n. Sesamia firmata sp.n. has atypical genitalic morphology, suggesting a strong selection resulting in a reinforcement of pre‐zygotic isolation. Some specimens previously identified as S. penniseti on the basis of morphology are sisters to S. epunctifera on the mitochondrial tree, and are connected to S. penniseti on the nuclear tree. The mitochondrial distance from S. penniseti and S. epunctifera is 7.6% and 3.9%, respectively, suggesting an ancient mitochondrial introgression from S. epunctifera into S. penniseti. The possible causes of the reinforcement and introgressive hybridization are discussed. This case of mitochondrial introgression, uncommon in Lepidoptera, in which females are the heterogametic sex, may be an exception to Haldane's rule. The hybrid is assigned the rank of species and named Sesamia pennipuncta sp.n.